The performance problems you're seeing relate to drivers. As of now, Nvidia is the ONLY manufacturer that has properly optimized drivers for Windows XP (and 2k for that matter). Like most hardware manufacturers, they've spent their optimization resources on garbage like Win9x, and haven't learned how to write proper WDM drivers (See Creative Labs for another good example). Also note that Nvidia's drivers are multithreaded, thus have improved performance on SMP machines.
No..I don't work for Nvidia, I just don't see anyone else writing proper drivers for WinXP/2k. ATI REALLY needs to get their act together.