Off the top of my head I am going to recommend XP. Officially it is more supported by M$ for compatibility with programs you have in mind. Granted, most drivers and software that works with 2K will work with XP and vice versa, it is not always guaranteed.
I've been using XP Pro for months now and I think it is great. Much better than 98SE. My only problem is that the new Creative SBLive! drivers refuse to install on this system. In fact, Creative has released the only software I know of that can reliably crash my XP OS.
There really isn't all that much difference between them unless you have your MSCE, then there's *all* kinds of difference between them.
Anyways, XP is a little better for gaming. I got such good resultz under Win2k, that it wasn't all that much better than XP for gaming. I would say that I have gotten several games that wouldn't work under Win2k to work under XP (without ACT installed).