What I've found with Win2000:
Half Life/Counterstrike runs great, if not better.
Quake III Arena runs ~5% slower and has brightness/gamma problems, even with latest Det3 drivers.
UT runs fine.
WinAmp will sometimes jitter, usually when there's heavy CPU/HDD utilization.
This is with 256MB of RAM and a Celeron 400, Yamaha sound - latest drivers for all. My home computer had the same problem with Win2k (where the gaming info comes from). I use Win2000 at work primary and Win98SE at home for gaming - I've switched between the two too many times to count.
G|T