Neither XP or XPpro is worth the money if you have 2k keep it and wait until vista (longhorn). If you dont have anything use Linux (at least until Longhorn comes out) cause you 'officially' cant buy 2000 anymore, though I am sure you can find it if you look..
I've heard that vista, now that they've taken out a lot of the beta features, is really nothing more than XP with a prettier smile.