The XPS 400 and Precision 380 are basically the same.
However most people get in trouble trying to use Joes Crab shack el cheapo ram which is High Density and therefore it does not ever work.

http://www.ebay.com/sch/i.html?_nkw=dell+precision+380&rt=nc&LH_BIN=1

 

The other difference is the 945 vs 955X chipset.   So contrary to FUD and myth Dell did not do this INTEL did.  Buying the Right 380 on ebay will also Yield a copy of windows 7 and a hard drive, Ram and other parts that are less than the cost of the windows 7 alone.

http://ark.intel.com/products/27720/Intel-82945G-Memory-Controller

http://ark.intel.com/products/27727/Intel-82955X-Memory-Controller

 

http://www.memorystock.com/memory/DellPrecisionWorkstation380.html

 

To clarify 2 Gig modules will work but you will not get more than 4 gigs due to the 945 chipset limitation.   The good news is that you can upgrade to a precision 380 and use all the parts from your XPS 400 in it.  They are very cheap on ebay.

ALSO Bios upgrades are usually Required for newer cpus and newer ram aka 2 gig modules. You will want to get a Pentium D 900 Series and the copper heatpipe heatsink.  That way you can upgrade all the way to windows 10 64 bit.

You can put up to 8 gigs in using low density NON ECC, NON Buffered, NON Registered  DDR2  256Meg x 64  2 Rank Ram. And the BIOS will see it but you wont get 8 gigs of useable ram due to chipset limitations.

4 GB with 32-bit Operating System and 8 GB with 64-bit Operating System after upgrade to Precision 380.  The Optiplex 620 has this same dilemma. The fix being upgrading to Precision 380 or Optiplex 745,755, 760, 780 instead.

If you are going over 4 I would highly recommend getting all ram from the same vendor.

The ram is installed in PAIRS 1 3 2 4 not 1 2 3 4.

Ram should be installed highest capacity to lowest capacity.

The ejector ears have a color variation to show you which dimm goes to which pair.

More than 3.5 gigs is not possible with 32 bit CPU.

DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 128Meg x 64

http://www.crucial.com/usa/en/xps-400/CT474084#productDetails

XPS 400 and the precision 380 are basically the same.

http://www.ebay.com/sch/i.html?_nkw=pentium%20d%20915

 

http://www.ebay.com/itm/Precision-380-390-T3400-XPS-400-CPU-Cool-Copper-Processor-Heatsink-Shroud-X9694-/131310107549

 

I have seen inexpensive Pentium D's on Ebay and Amazon

SL9QQ seems to be the max for a 5150

Intel Cpu Pentium D 945 3.4Ghz Fsb800Mhz 2Mbx2 Lga775 Dual Core Tray

Intel - P-D 3.4GHz/4M/800 Dual Core CPU SL9QQ 

6 used from $12.00

SPEED STEP THANK YOU !

I did not have access to 380's. I was aware of some limitations with both of these units. Knowing of WIN10 in the very near future, with that knowledge of end of service life with WIN7 these machines should get plenty of service life. I have already updated the processor to a 950 and improvement has been seen. The problem is memory.

I am going to try different memory modules to see if that fixes my issues. Previous Dell machines that were in service purchased the RAM from Dell directly with NO problems. This was the first from Amazon. Will use either Crucial or from suggestion Memory Stock. In your past purchase history which of these two have you used the most and best service (purchase and performance) with ?

Thanks Again, look forward to further comments.

So I can understand, you mentioned that 2GB x2  modules will work.  Is this correct ?

I am at a loss, so looking for some insight on a direction.

I installed a matched pair of ADATA PC6400 in a 5150 (similar) with excellent results.

Update for this post,

Crucial memory was the trick. I have had issues iwth some non off brands but on the last Dell I built, there was none. Installed 4, 1GB 5300 sticks and was seeing 4 GB RAM.

I also in the middle of all of his I  updated processors for both machines. I updated all the way to 3.5 and found significant performance increase (6.3). I see that both of these units will go al the way to 2020 with out a issue. 

Thanks for the suggestions from everyone.