All the computers I use have Win2K on them, generally the server versions but thats simply for web or work related business. I can't guarrantee how games will work on that platform but I'd go for Xp regardless. Its best to get yourself a copy and dedicate a whole day simply to putting on patches and covering up those annoying security holes.
When Xp is up to date, it runs perfectly just beware that its rather buggy in its "vanilla" form. I've tinkered with Xp in the past, generally on the campus computers and there's an option to make it appear like the old 98 version but its very dependent on memory, so its not exactly the best thing to throw on an old PC. Unless your planning to run an intensive network don't bother with 2000, grab Xp and spend a day at the Windows Update page.
