As a student, I decided to update to Office 2004 because I saw the features and price to be worthwhile. The newest features that I use most frequently are the compatibility reports, the (what seems new) thesaurus (really helps for papers), the project center is nice, Unicode support, and a little bit more support for fractions (and that's just in Word). I don't know how fast your PB is, but it does seem a little bit smoother on G4's (at least at CompUSA on their iMacs). I haven't used Excel or PowerPoint extensively, but they seem to be nicer too as far as features goes. The whole suite seems a little bit more "complete." Depending on how much you use Office, I would suggest upgrading or not. If you use it every day, I would upgrade, but if you use it once in a while, I would suggest staying with v.X unless you have the extra cash and/or see the new features to be worth it.
Chase