I also run with an 8800GT, 4gb ram, Athlon 6000+ dual core, and am using windows XP 64 bit.
The game runs perfectly fine with all settings at max.
Sounds like something strange is happenng on your PC.
All I can suggest is the usual stuff, make sure you have the latest nvidia drivers (182.50), get the latest directx from microsoft (www.microsoft.com/directx), and install all the windows patches (windowsupdate.microsoft.com).
Maybe try booting without other resident processes (start->run->msconfig) and if that improves it, use trial and error to find out whats conflicting.