Just a sugestion, but have you tried the detonator drivers from NVIDIA - I know it's not recommended for 3rd party cards, but ...
I have a Geforce2 MX (AGP 4x), which I picked up cheap a couple of years ago from some 3rd party company I'd never heard of. The NVIDIA detonator drivers improved the performance of my card significantly (about 40% faster than the drivers supplied with the card), and have worked fine ever since. The latest version of the detonator drivers work correctly as well.
The only problem I had on my Win98 system was to get the drivers to install correctly.
I resorted to completely removing all copies of the original drivers, booting in safe mode (i.e. VGA), and then running the NVIDIA driver install program which did the trick for me ...
Don't know what the equivalent approach would be in XP, I'm afraid.