XP has a lot of newer features not in 2000 or 97.
XP allows voice recognition both in speak to type, and speak to use menus/commands.
XP has more organizational charts, etc.
The window becomes divided into panes. Down the right hand side of the window is a new "task" pane, which changes based on current task. For example the File/New window is displayed there, not in a dialog box. The clipboard also shows it contents there.
When you draw new objects using the drawing tool bar a "drawing canvas" is created to help keep them together, etc.
Excel XP has cool new items in the AutoSum button, it can now do all of the things that Autocalculate could do.
PowerPoint XP will allow more than one design template per presentation (used to have to link presentations in prior versions).
And more that I can't think of now. . .
Most of the new features in XP are "Bells & Whistles". Not anything that most people "HAVE TO HAVE".
Lots of them are fun new things to play with.
I would agree with DreamBoat - If there is something specific that you haven't been able to do, that you know can be done in 2000 or XP, then yes go for it. If not, I am one of those people who likes to stay in my comfort zone.
I currently have all three versions at different locations I use.
Good Luck,
AngO