Random updates, security fixes, and general tweaks. Nothing too fancy. For a well-maintained system, SP2 will suit you fine. Many people prefer it over SP3, myself included.
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...).
I had a heap of problems with Automatic updates when I installed SP3 so reverted back to SP2. This may have been sorted with recent updates and I will go down that route again soon. Having the latest should/must be better.