Lookout JP-looks like another post going to the viagra/hard drive level

(but, hey, the chicks dig it, huh? )
Volunteer: XP is quite an improvement on 98 (IMHO). Better initial driver database, and generally better performance and stability. And, as JP way saying-System restore is a great thing, not to mention the remote assistance feature. If you're not sure whether or not you want to upgrade, you can always run dual O/S's until you decide which one you like better-chances are, you'll be booting to XP more than '98.
As far as problems switching motherboards-you'll have to contact MS to reactivate windows after too many system hardware changes(which irritates me to no end-but a necessary evil-kinda like Rick

), but it doesn't take very long to do.