Windows XP
---------------
Looks good (fonts especially), and has lots of new toys and several new features, increased games compatibility. But is potentially unstable and can perform moderately poorly even on powerful machines. Don't let that put you off, though, because its a delight to use if it likes your machine (which it may well) and its reached SP1 now which has helped things alot. Could be better but it will improve, and its not bad right now.
Windows 2000
------------------
Runs well on a wide variety of PCs, even older ones to a certain point (check minimum required specs!). Has matured a great deal from its beginnings, and has all of the core technologies that one needs, in terms of workplace LAN, small office home office, etc. Becomming increasingly secure (well, as far as MS is ever going to be) and is tried and tested now for several years. Getting to a 4th service pack indicates a fairly mature Microsoft NT based OS. NT4 was pretty solid by SP4, and Windows 2000 is now too.
It all depends, really, on what you use your computer for. Both platforms have their good and bad points. And in many ways, they are also the same thing. A lot of security fixes released now apply to both operating systems and they share a lot in common.