I have dual 512MB NVIDIA GeForce 7950 GTX cards.....
But seriously, I don't know much of anything about ATI Radeon, since I've always stuck with Nvidia. Someone posted a while back about a problem with a video card doing something similar to this though. Noel suggested something about the card may be overheating, or the card may just not be able to handle the graphics. I don't know. How does your card perfom with other games with next-gen graphics? The same, better, worse?