"If it ain't broke don't fix it." If your system is running fine and never crashes, keep it that way. On the other hand, if your system is unstable you might as well upgrade to XP.
From my own experience, XP very rarely crashes and it has a lot of neat little bonuses that you'll find here and there...also it is very customizable which is always nice. But if you have no need t upgrade...then why bother?