Personally I hardly ever play games and when I do, it is mostly simple games that do not require 3D.
I chose Windows 2000 Professional for its stability. I used to run Windows 98, but I really got fed up with rebooting at least 3 times a day because of memory leaks and other crashes. In the beginning (bought win2k when it was released) there were many problems regarding the drivers of hardware and patches for software (especially games).
At this time most products (not older than 2 years, for older products it just depends on the manufacturer whetehr they will support it) run under Windows 2000 after installing the latest ServicePack.
As Mattel said, it is no OS for playing games, for this you are better of with Windows 98 or perhaps Windows ME. Windows 2000 is the successor of Windows NT and should be seen as such. It is a very stable OS, at this moment it has an uptime of 3 weeks (plus a day), which I never got when using Windows 98.
It did cost me a lot, but from my perspective it was all worth it (yes I know, probably one of the very few here that actually buy stuff, so sue me )