Re: Xorg freeze on Ubuntu 7.04 with 6800GT
If disabling AGP eliminates the problem then this is either an AGP driver bug, or a hardware problem. You could try testing NvAGP with other values to see if they work, as well as testing with a lower AGP rate.
I couldn't speculate on why this works in Windows. Its possible that they worked around the problem in the Windows AGP driver, or it could just be an AGP driver bug in Linux.
Regardless, this doesn't sound like an nvidia driver bug.