Loss of peripheral power after upgrading processor

I'm in the process of upgrading my rig. I had:

MSI P6N Platinum mobo
E4400 processor
GeForce 8500 video card
a couple sticks of 1Gb PC5300
and a couple hard drives, DVD-RW and the normal USB items.

I'm slowing replacing stuff. I started by replacing my video card with an N250GTS. All went well.

Next, swapped my processor with a Q8200. Everything seemed fine, but the comp wouldn't boot. Finally figured out that the HDDs weren't spinning. Then realized that none of my peripheral 12v had any power. When I put my E4400 back in, all is good. When i put the Q8200 in, 12v peripheral power is lost.

I have a nMEDIAPC MP-500 500W ATX12V Power Supply. Where do I go for troubleshooting from here. Everything I read saus my PS should be able to handle the load. Could the CPU be bad?

i didn't at first. when the q8200 is in, my computer nevers sends a signal to my monitor, so i couldn't make bios adjustments. at one point, i did throw my core duo in, got into the bios, changed the fsb to 1333, saved the settings and powered the computer off before windows loaded. i wasn't sure how damaging that move might be to a CPU with a 800 MHz fsb.

so when i swapped out to the q8200, it was the same deal. i thought maybe the fsb setting change didn't work because of how i powered it off. however, after swithing back to the core duo and going to the bios, the fsb was still on 1333 MHz.

i ran the MSI update utilities and my bios is current.

just to add more... the new cpu never heats up. bad cpu possibly? is there a full proof way to tell? too weak of a power supply? is there a way to determine that?