Ok, I'm now posting this response (as well as POSTing ) on my main computer. I switched out my E8400 and put in my old E6320. The 6320 worked in BOTH motherboards (P5N32-E SLI, and the P5B Deluxe).

So you'd think that my E8400 was shot. It's not.

I had taken the 6320 out of a file server I had been running. When I put the E8400 into that file server (AsRock Wolfdale1333-D667 motherboard) it works perfectly. But no matter how I configure the BIOS settings or reset the CMOS, it will not POST in either my P5N32-E SLI, or my P5B Deluxe.

Any ideas? I don't particularly want to have to permanently downgrade my main computer's CPU

I tried both my old Enermax PSU, and a spare PSU I had around (the one that powers my file server, from which I have posted every response in this thread except this one, so I know it works).

The new OCZ PSU seems to be working just fine now though...

Hmm. Maybe try for a BIOS upgrade, it could be a BIOS issue, although that seems unlikely as it was working previously.... hmm.

Maybe core voltage is off? Both boards with which the E8400 refuses to work are ASUS? Check what voltages that are supplied to the processor - honestly, gotta be some common ASUS issue.

I had already updated to the newest (1701) BIOS for the P5N32-E SLI, and 1701 is still the newest one available

My E6320 runs at a higher voltage than the E8400 stock, so I went into the BIOS on both boards and forced the voltage to run at exactly stock for the E8400, then switched out the 6320 and tried to POST with the E8400. Not functional.

EDIT: I can't seem to find much on the internet that could explain why the CPU works in my server but not in my other two motherboards. I'm really at a loss... Maybe I should just go buy a new CPU?

Update: I've found that while the E8400 will POST on my AsRock motherboard, it will only do so using the motherboard integrated graphics. As soon as I disable the integrated graphics and attempt to POST with any video card, I will get the usual result of a blank screen and no POST.

At least this tells me that the reason my E8400 wouldn't POST on my P5B or P5N32-E SLI was because they don't have integrated graphics, and must use video cards. However, I have no idea why this might be happening or if there is anything I can do to fix it. Any ideas?

If that's the case, it sounds more like a PSU issue than anything else.
There's no reason for a certain Processor to inhibit a GPU to work, so it seems there's a power shortage.

Baffling. Check what are the power consumption specs for E6320 vs E8400. I would assume E8400 being a 45nm vs older 65nm E6320 is less power hungry. Both are 65W designs though. As a last test, try updating BIOS. It's just really weird, unless E8400 requires significantly higher amp rating on certain voltage, I see no reason whatsoever that PSU protection should kick in on three separate PSUs (if I'm reading your posts correctly). 65W is peanuts. If BIOS update doesn't work, I would go with redk's suggestion - try another PSU.

Thanks everyone for all the suggestions. I'm still trying to figure out the mystery of this E8400, but in the meantime, I just broke down and bought an E7400 which I overclocked to 4.03 GHz, making it faster than my old 8400 anyway.

If I find a solution, I will post it for the sake of completeness.