Need help diagnosing problem

Recently I have run into a problem with my computer booting up. When I turn the power on it will run all the lights and fans, hard drives, etc, but nothing will actually boot up, it will not post or produce anything on the screen, though it will activate the auto turn-on of my monitor only to have it tell me there is no signal.

It all started while I was playing the MMO Vanguard (system req's for this game are very high so it was running my computer pretty much at 100% at all times). While i was playing the game all of a sudden became very choppy going from 30+ fps to 1 or 2 fps, then totally crashing and at that point I was not able to turn the computer back on, again it will light up and run the fans, hard drives etc.

I believed it was the CPU or motherboard because upon testing the CPU in a friends board it crashed the system and wouldnt boot up again (I had to buy the friend a new mobo). The motherboard would not work with different components either, I also tested the graphics card in another computer without incedent so I'm sure that is not the problem.

I have since purchased a new mobo and CPU and I ran into the same problem, everything lights up and spins up but there is no display in the screen (I do know the monitor works, I have tested that as well). My next inclination is that the power supply is bad but wanted to get some more opinions. Thanks in advance for any help.

specs:
CPU: AMD Athlon X2 3800+ skt939 (old and new)
Mobo: Gigabyte GA-K8N SLI (old) || ASUS A8N32-SLI Deluxe (new)
Ram: 512mb x2 Kingston HyperX PC3200 ultra-low latency
GFx: MSI 6800GT x2 in SLI
sound: Creative Audigy 2 plat
HD1: 18gb Maxtor SCSI 15krpm (OS) (w/internal PCI SCSI card)
HD2: 80gb WD SATA (games)
HD3: 250gb WD IDE (storage)
power: Antec 480watt Trupower

I bought a completely new processor, my old one was out of warranty being OEM from newegg.

Glad you found it and have your system up and running.

Yes, I did run into some problems when I first put the 3800+ X2 processor into the board, it would not recognize both cores. Make sure you do incremental bios updates, dont skip bios versions, just upgrade to each consecutive version then the next. Once I figured that out I had no problems, it would recognize both cores and I didnt have any issues until the problem with the power supply. Which I now suspect went bad because I overstressed it upgrading to SLI.