I think only you can decide whether to upgrade or not, but here's a Microsoft web page that compares Office 2003 with Office 97, Office 2002, and Office XP.
I use Office 2002 and whilst it keeps doing what I want it to, I don't think I will upgrade. I expect that I will have to at some time though, especially when Microsoft's new Operating System Longhorn is finished, but that looks like a year or two yet.
what is the difference between these two programs? Should I update to 2003?