Windows 2000 is Windows XP, minus all the unecessary, memory hoarding things. Either way, it's still an upgrade, and they're both thousands of times better than ME.
XP has a lot more technological things; but 2000 is cheaper and does the job fine. Only a few programs that can go on XP can't go on 2000 and that's mainly because of the tech stuff and there're generally Office or Windows Media Player... that sort of thing.