As with any upgrades you should test in a representative testing environment or pilot on a few clients first. Other than that you probably wont get much more of a response without some actual information in the original post to clarify what you need to know...
So you have Windows XP now, with Office 2003 installed?
What version of the EV Outlook Addin do you currently have installed?
What are your (rough) upgrade steps going to be?
Is anything else needed on this issue?