390 chipset H61 in dell 390 bios doesnt support 3rd gen.

Core I7 2600 is max

Intel says chipset supports 3rd gen but only specific models.

So I would check what bios version is with old cpu and update to latest BEFORE trying the new cpu.

https://ark.intel.com/content/www/us/en/ark/products/52806/intel-h61-express-chipset.html#tab-blade-1-1

@smallChomosuke 

You mentioned that you put back the old CPU and it is still not working.

I hope you practiced good ESD protection or you can very well have zapped your motherboard.

.

When was last time your 390 was working?  if you put back in the old cpu and no POST, you can try take out CMOs battery over night, replace next day and reset CMOs jumper.

Sandy Bridge chipsets, except B65, Q65 and Q67, support both Sandy Bridge and Ivy Bridge CPUs through a BIOS upgrade.  It is not certain whether latest Dell bios for 390 includes update for ivy bridge (3rd gen intel core).  you should check your 390 bios ver. when possible, but your most pressing issue is no POST now.

As soon as I put it in my i5 3470s that's when nothing would show up on my moniter but all the fans start spinning really fast Including my GPU fans.

I refuse.

fans spinning does not mean much.  according to anecdotal reports the 390 bios is not updated to support ivy bridge cpu (3rd gen) although the chipset is technically capable.  Dell does this quite often to force user to buy newer models.

I wonder if the newer i5 is much of an improvement over the sandy bridge version?

Hoping you'll at least be able to get the old cpu to work again.

ditto.

Were you able to resolve this?

I hope so because I have the EXACT same problem. I am trying to  put in the I53470 (no S) CPU into my Optiplex 390 SFF and everything seemed fine. I checked a few sites for compatibility and didn't see issues but apparently there are.

I can't get the old CPU to work either and now I can't get to BIOS at all or anything. Super frustrating.