Practically speaking, both OSes are about the same. They are both built on the NT kernal and are both very stable. Windows XP is newer and will be supported (hardware and drivers) more than 2000, but I seriously doubt 2000 driver support is going to slack too soon. If you are going to be playing older games, I would go with XP, as in my experience, XP handles them better than 2000. If mostly you will play newer games, either will be fine. To address a few comments: Activation is a non issue if you asked me. You can install the OS 3 times in 3 months before having to pick up your phone and spend 5 minutes with a MS rep. You could also install Windows once, create a Drive Image/Ghost backup and restore it when you have trouble. And as for "eye candy". If you don't want/like it. Turn it off. A couple mouse clicks will fix that and you will essentially be using a newer version of Windows 2000.
\Dan