Very little, to be honest about it. Most of what's in SP1, you already have, or have been offered. If you need to do a clean install, all you have to do at the first check with Windows Update is just allow the single update KB976902 to install, and only that update. Then install SP1. Of course, you install your drivers manually.
After that, you just have your monthly updates to apply, and you're done. No need to apply those over 200MB of updates first, they're in the SP. I've done this twice, once on a regular install, and once in a VM, they both worked fine.
As far as the real benefits goes, don't expect a miracle. From startup, it knocked about 10 seconds of wait time (for me) to connect to the internet, it's also been reported that there are improvements for Windows Virtual PC (however, I couldn't tell it). And I use it daily enough to know.
I feel that the largest beneficiary of SP1 will be M$. The update that was offered that many refused (KB971033) will be there, a piracy deterrent, and the many (home & business/corporate) who refused to adapt until SP1 is released will get over their fears and finally make the dive. MS will gain many customers over this release, and you'll soon begin to see a more steady decline in XP usage (they've already fell to 45.3% market share). That's over a 30% drop since their peak of 76.1% in January 2007. That also means that XP doesn't fully control the market any longer, since they've fell below the 50% mark, a critical point for the aging OS.
The supremacy war between Windows 7 & XP will really heat up in post SP1, there will be many promotions, incentives & gimmicks (trading in your old) to buy new computers, and they will move.
To this date, XP has been MS's hugest success, at their peak having over 400 million users. Most likely, when Win 7 hits it's peak, those numbers will be doubled. SP1 will be instrumental in making that happen.
Cat
