Ok, I've recently built a new rig that goes something like this:
Intel E8400 Wolfdale
Asus P5E3 Deluxe WiFi-AP @n Edition
4GB OCZ 1333MHz DRR3
850W Zalman PS
Visiontek 4870 (Times two when I ran Vista)
Windows XP SP2
My friend who built his a little before me has:
AMD Phenom X4 9600
Asus M3A32-MVP Deluxe Series
4GB Corsair 1066MHz DDR2
1000W Ultra PS
Visiontek 4870 (The newer one with the different heatsink)
Now Windows XP SP2
My computer went together like cake, his has always had trouble. Everytime he tries to play a game it shuts down and reboots. Now, the thing I don't understand is he had first bought two Visiontek 3870 X2 and returned them because apparently they all had bad BIOS and would shut down after about 12 minutes from overheating. So he goes and buys a Radeon 3650 and it plays like it normally should. So he updated to what I bought, the Radeon HD4870 and now it won't play games but it's running 58 degrees C according to Catalyst while on the desktop. It can't last 10 seconds in game play. I even switch out my card and put it in his computer and same result.
Everything is up to date because we've reformatted his drive and installed everything the way I do mine. No overclocking, no extra tweak programs, all updated drivers and BIOS a fresh copy of Windows XP and all cables are hooked up correctly.
I think it's a power problem but he has both 6pin PCI Express cables hooked up to the video card.