I think there has been some problems with the dual core AMDs combined with the Nvidia drivers. Check the Hardware forum for the sticky.
Second, I had nothing but problems with the 81.98 driver... low frame rates and different frames depending on where Iwas looking. Might I suggest the 81.85 driver?
I use WinXP, I have Win64 on a separate boot, but I don't log onto AH with it. Maybe 81.98 runs better on 64, I don't know...