I personally would get it just for the sake of having a completely up-to-date system (I actually did, but my XP rig got bricked so now I'm running Ubuntu on my old P3 rig
). I did notice that my boot times got a lot faster with SP3; however, I had this weird problem when svchost would sometimes torture the system (100% CPU usage); but that seemed to resolve itself. They had some minor issues at the release but they were fixed, and installing SP3 shouldn't introduce any new problems, it may even fix some you're having now (if you are, that is...).