I have been running into this problem lately more and more.
Every so often playing any game my comp will totally lock up and I've narrowed it down to it being an Overheating problem.
My card isn't overclocked at all, but during times of play I notice that I'll see a rainbow pattern in certain textures.
eVGA nVidia 5900 Ultra (Latest Drivers)
x2 80GB Western Digital HDD's
3200 AMD Athlon CPU
Asus A7V8X-X Deluxe
I have a case fan that blows directly onto the video card, and 2 80mm ball bearing fans to aim directly at the card and yet my computer still locks up.