You would be better off performing a clean install of XP rather than go through all the hassle of uninstalling Vista and then wondering if its all ok. If you plan to install XP anyway, do it cleanly.
Backup all of your files etc though, even your emails and internet favorites.
Getting connected after shouldnt be a problem, i assume you know your user pass etc. You can always phone your ISP anyway for help getting online after.
Your XP disk, depends on whether its a retail or OEM, if its retail go ahead and install.