Coming in at less than 350MB, the service pack includes fixes for over 900 reported problems, some of which have already been resolved with post-Service Pack 2 hotfixes.
Some additional info for Vista SP1 is that it has yet to delivered as an update to testers and is being delivered as the full ISO. Claimed ISO sizes are 3.07 GB for the 32-Bit edition and 4.3GB for the 64-Bit Edition. Note that the large sizes may be explained by debug code for this early phase of testing. Also claimed are that most of the files carry the 6001 build tag
I read that the final versions might be available first half of 2008, but this ZDNET author seems to speculate they might release XP SP3 as early as this fall. I wouldn't install either one of these alphas as of now, but it sure makes me anticipate the service packs.