Kinesis:
Functionally Win2000 and WinXP are very similar.
There are a few features Win2000 doesn't have though.
If you think you'd use it the new UI is pretty spiffy

I personally wouldn't likely take the time to play with it, but if you are the kind of person that skins winamp imagine now you can do it to your whole OS

(if you don't like it you can turn it off).
System Restore, again not something I want (you can turn it off to), but for a lot of people it's a very handy feature.
WinXP also supposedly has better support for Digital Cameras.
I've heard that WinXP has better legacy compatibility..but I've also heard Win2000 can get that to (I'm downloading as I type this).
WinXP of course comes with IE6 and WMP8, but you can get both of those for Win2000 as well.
Now you realize I'm comparing XP to 2000....they are both in the NT5 family...if you compare XP to Windows 4.x (95,98,ME) there are huge lists of things that are better about them.
Search around...there was a thread a while ago about WinME vs Win2000.
Trust me, you'll want to be at Either Win2000 or WinXP. The NT5 family is a really really solid OS.
As to which..if you are on Win4.x just wait for XP. If you are at 2000 already it's a harder choice (that's where I am).