Not using xgl (I just checked apt and the xserver-xgl package isn't installed).
I know the 3000XP is a tad on the old side, but doesn't seem to be the problem here. If the CPU were the bottleneck, the frame rate wouldn't decrease with the resolution, while I can still get high frame rates at 800x600.
There's a review here (http://dansdata.com/upgrade.htm
) which shows significantly better performance on a 6800 than my 7800!
Since nothing obvious has popped up that might cause this, I've attached my nvidia-bug-report.log below, it seems to have pretty much everything that could be relevant.