I agree on a couple of the posts above,,,
first off,, I ran win2k pro,,probably from the second day it came out until XP was released,, and win2k pro has many favorable options and abilities,, however, there were some drawbacks with drivers specific to NT format,,
with the move to XP,, my first XP box was an old Dell PII 400.. that sucker ran so well and so stable( over 65 days without a reboot),, that I was soooo impressed with XP pro,, I went out and bought an upscale machine to really use the OS to full adavantage,,
I would have to go with XP pro at this time,, good , fast , stable, reliable,, however,, it DOES REQUIRE simple maintenance,, and that is where some of the win9x features are a pain in XP,, cleaning up system restore points, regular defragging, disk cleanup,, are MUSTS,, not just something you do, as in win98...
plus,, i have always had luck with the compatibility mode in XP,, even gaming,, (eventually worked well),, so i would say that it all depends on the game architecture too,, not just the OS,, but the combination,,
Go XP,, with min 512 mb of ram (GOOD RAM),, and you will have a positive experience,,
cheers,,