I've done it before as well, and after your restore from a ghosted image you don't have to reactivate. You generally only have to reactivate with a clean install or a significant hardware upgrade.
Also keep in mind that the activation info seems to get flushed from the MS database after about six months or so, so you usually only have to call MS to get an activation code if the same copy of XP is activated more than once with a few months. After a longer period of time, the online activation typically just goes through automatically without having to call, at least in my experience.