Intel Xeon X5687 in a Dell Studio XPS 435mt?

Its nowhere near simple. Bios should be updated to latest before trying IMHO.

Microarchitecture    Nehalem
Platform    X58
Processor core     Bloomfield
Core steppings     C0 (SLBCH)
D0 (SLBEJ)
CPUIDs    106A4 (SLBCH)  106A5 (Q1H7, SLBEJ)
Manufacturing process    0.045 micron

********************************************************************************************
Core Model Type/ProcNo.     W3680
Core Stepping     B1
CPU ID String     206C2h
CPU Socket Type     LGA1366

*********************************************************************************************

Dell Studio XPS 435T/9000 (Tower)

I'm typing this from a Dell Studio XPS 435T/9000 (Tower) with a Gulftown / Westmere EP Xeon W3690 (6 cores, 3.46 GHz). 

The reason this CPU works in a 435T is because Dell added Gulftown (and by extension Westmere EP) support with one of its last BIOS updates (A15 BIOS).  Note the "Update BIOS to support new intel CPU (Gulftown) launch" under the A15 "Fixes and Enhancements".

Gulftown / Westmere EP Xeons include the W36xx and X56xx series.  The W3690 is the fastest in its series and I'm using it with my 435T, so a 435T should be good with any other processors in the W36xx series.  The W3690 seems to be the fastest Xeon hexcore for the 435T (equivalent to a i7-990X).

I have not tested the X56xx series in the 435T.  However, it you go through the benchmarks on userbenchmark.com for the 435T/9000 here you'll find one running the X5687 (fastest in the X56xx series).  This one.  So, it looks like the X56xx series should work with the 435T.  The X5687 seems to be the fastest quadcore that works with the 435T.

I've also tested the 435T with the Xeon W35xx series (Bloomfield), and can confirm the W3580 (fastest in the W35xx series) and W3565 work in the 435T.

Dell Studio XPS 435MT (Mini Tower)

On the other hand, Dell never updated the Studio XPS 435MT (Mini Tower) BIOS to support Gulftown / Westmere EP.  This limits the 435MT to Bloomfield processors, which only go up to 4 cores. 

For the 435MT (Mini Tower), any Bloomfield processor should work.  Best you can do is a i7-975 or its Xeon equivalent, W3580, both are quad core at 3.33 GHz.  Any of the Bloomfield Xeon W35xx series should be ok with the 435MT. 

I do have a 435MT (Mini Tower) which has the above mentioned Bloomfield Xeon W3580 in it, so I can confirm this works.

For reference, below are approximate prices (from ebay) of top CPUs that will work with the Dell Studio XPS 435MT (Mini Tower).  All are quad core.  Note for the same performance the Xeons are a lot less expensive:

Bloomfield, i7-9xx series:

i7-975, 3.33 GHz, Unlocked Multiplier - $90 (wide variance in prices)

i7-965, 3.2 GHz, Unlocked Multiplier - $70 (wide variance in prices)

i7-960, 3.2 GHz - $50

Bloomfield, Xeon W35xx series:

W3580, 3.33 GHz, Unlocked Multiplier - $30

W3570, 3.2 GHz, Unlocked Multiplier - $20

W3565, 3.2 GHz - $10

Without overclocking there's no practical difference in performance between the 3.2 GHz processors when used with the 435MT.  For example, buying the i7-965 (which has an unlocked multiplier) over the i7-960 provides no advantage for the 435MT without overclocking.  Also, there's so much bus bandwidth with only one 16x PCIe slot in the 435MT that 4.8 GT/s QPI vs. 6.4 GT/s QPI bus speed doesn't matter.  Same with W3570 vs. W3565.

However, since the 435MT can now be overclocked if the CPU has an unlocked multiplier, that could be a major factor in choosing between one of the 3.2 GHz processors.

HWiNFO64 screen capture showing details of a Studio XPS 435MT (Mini Tower) with a Xeon W3580 in it.  Xeon W3580 is shown in upper left hand corner.

This machine has its original 360W PSU.  In addition to the Xeon W3580, note it's running:

• SAPPHIRE Radeon HD 7850 2GB 256-bit GDDR5 Video Card, model 11200-00-20G (the Non OC version, which runs by default at Reference 860MHz Core Clock).  This GPU is pushing the limits of what will run with original 360W PSU.  A GPU model that runs at low reference clocks was explicitly chosen to make sure it would work with the original PSU.  I've never tried to OC the GPU on this machine.
• 24 GB memory (6x4GB Kingston HyperX KHX1600C9D3/4G running at 1066 MHz and CL6).  Even though the CPU and RAM support 1333 MHz memory speed it's only running at 1066 - shown on HWiNFO as 532.0 (1066 divided by 2) - due to motherboard and/or BIOS limitation.)
• Windows 10 Creators Update, Version 1703 (shown in HWiNFO as Build 15063.674)

Note there are other users running the 435MT with W35xx Xeons, seen by looking at user Geekbench benchmarks on Geekbench Browser:

W3580, 3.33 GHz - Geekbench 3, "Dell Studio XPS 435MT W3580" benchmark search - 3 results found as of today
W3570, 3.2 GHz - Geekbench 3, "Dell Studio XPS 435MT W3570" benchmark search - 2 results found as of today

In addition to the Bloomfield Xeons, I suspect the Gainestown 55xx series Xeons might work as well in the 435MT.  Both use the LGA1366 CPU socket.  Gainestown came out from Intel about the same time or just before Bloomfield, so support for it is likely in the microcode for the 435MT.  To my knowledge, no one has tried a W55xx yet with the 435MT. 

Top end processors for Gainestown 55xx series and approximate prices on ebay, all have locked multipliers:

W5590, 3.33 GHz - $30, wide price variances
W5580, 3.2 GHz - $25, wide price variances

I'd probably stick to the W35xx Bloomfield Xeon series for the 435MT, especially since the W55xx Gainestown series all have locked multipliers and can't be overclocked in the 435MT.  But, the W55xx series would be worth a try if cheap and/or the W35xx series isn't available and overclocking isn't a factor.

I'm not seeing your prices.  W35 series is available but not at the $30 mark

https://www.amazon.com/gp/offer-listing/B002JVWCOU/ref=dp_olp_0?ie=UTF8&condition=all

Intel SLBET
Intel Xeon W3580 CPU Processor
Intel AT80601002274AB
Intel BX80601W3580
3.33 GHz, 3200 MHz, Socket 1366, 8 MB, 130W

Ebay however has shady dealers that steal your money.   I ordered 8 units at $29  for a total of $232 and they said back ordered after they took my money.  Then they split the original order into 2 and 6.  Then they waited another week and shipped 2.   Then they said out of stock.  They then re listed the same part at $59 each and said I had to re order if I wanted the other 6.   I disputed the entire charge with AMEX.   They have yet to contact me about the difference.   Ebay was not helpful at all.  This is why I will not tie my ebay account to a bank account because they will ALWAYS Debit your bank account so that they don't have to refund your money.

Its been about 4 weeks now and I don't think the Ebay seller will try to work things out.    I haven't used the 2 that I got since I may still send them back to the vendor.   Sellers remorse is rare but happens.

As I mentioned, prices were from ebay, not amazon.  Shop around.  ebay is almost always cheaper than amazon.

Here's a W3580 that sold 2 days ago on ebay in the U.S. for $16.06 with free shipping.

And here's a "Buy It Now" current listing for a W3580 on ebay in the U.S. for $28.00 with free shipping.

Not sure the seller is shady.  The seller may have sold the 6 through another selling venue and not been able to update the quantity on ebay before you bought it.  That's understandable.  But, would be nice if they honored the original price.  But then again, they may no longer be able to source them to make a profit at that price.

I'd take the 2 at $29 and shrug my shoulders at the rest (6) - meaning not try to get the additional 6 from that seller, but cancel the order for 6.  First I'd take the seller's communications saying they're not going to deliver the 6 at the originally agreed upon price to ebay and ask that the price for 6 be credited to your card and order for 6 be cancelled.  I think given seller's communication ebay would see that as totally reasonable.

Then I'd just buy the additional 6 you wanted elsewhere.  But, how you handle it is up to you.

That's the issue.  They charged me for 8 and shipped 2.  Then tried to delay me past 30 days so that they could keep my money.  Amex reversed the charges since I only made one order.  Because of the bait and switch order splitting there is no order to cancel.  The ebay person was clueless when I said this wasn't acceptable that I was charged $232 for 2 cpus when I ordered 8.  There were more than 25 listed as available from the vendor.   2 pieces at $116 each is not 2 @ $29 each with free shipping buy it now.

They also promised a refund in 2 days for the "missing" parts but this of course never happened.  I gave them 20 days to sort it out and then disputed the charge with AMEX.  They promised to ship within 2 business days.  Then back ordered.  Then split order into 2 NEW orders that I did not make.  Basically they are gaming the ebay system trying to time you out after 30 days and say well its your tough luck we got your money even though we promised 2 business days.  I was fine with it being late after 10 business days but not after they started splitting orders and other nonsense.

If they didn't want to sell me them at that price they should have just cancelled the entire order and refunded my money.  I knew there would be problems when they shipped 2 of 8 and then emailed "order complete".   They were hoping that I wouldn't notice the full amount charged for 2 of 8 delivered and wanting to delay me for 30 days so that they could say I waited too long to complain.

@techgee - You seem to be the experts with these systems. I just got my W3690, only to find out it will not work in my 435MT. Is there no hope? I think I got too excited after reading an article for the 435T. Trying to eek out any increase in performance.

SpeedStep, I'll keep your experience in mind next time I purchase multiples of an item from a seller.

jcrick, yeah it's easy to get the two different Dell 435 models mixed up.  Hex cores like the W3690 in the 435MT (Mini Tower) are a no go, unfortunately.  I tried a i7-980 with no luck.  Realized then that since Dell didn't update the BIOS to support Gulftown CPUs that it was never going to work.  To maximize performance, I'd go for a 3.33 GHz quad core - W3580 or i7-975.  Best I've found that works.  

Now if I was a real expert I'd be able to update the Intel CPU microcode in the BIOS to fully support the hex core Gulftown processors.  ;)

All the Bloomfield CPU's will work - I run i7 975x (you don't need the x processor)  -  you can get i7 975 ebay as low as $39.00 today (3.33 ghz)

Just so we're clear, $39 only buys you an i7-975 described as “bad cosmetic condition, oxidized pins, lost capacitors. boots to Windows 7 system successfully, sell for parts or not working. Sorry no return on this item" from a seller in China.  The trending price on ebay as of today is $94.33, although as edward4691 has pointed out prices vary widely. 

I thought I had read earlier that the only way to get the 435MT to support 1333 mhz RAM speed is by upgrading to the i7-965 or i7-975 extreme edition CPU with 6.4 GT/s QPI. Since this is faster than the 4.8 GT/s for the i7-920 & i7-940, it supposedly increased the RAM speed to 1333? Was that only a myth? Can anyone confirm or deny if that's true with the 435MT? And if these extreme chips don't increase the RAM speed, is there any benefit at all from the increased 6.4 GT/s QPI? Does the 435MT lock it to 4.8 GT/s regardless? And does the same apply for the equivalent Xeons?