It's the second Tuesday of the month...
MS04-11 - Windows fixes
MS04-12 - RPC/DCOM fixes
MS04-13 - Outlook Express fixes
MS04-14 - JET fixes
Apparently Windows XP SP 2 already contains these fixes (Windows Update tells me there's nothing to do), so if you're running RC 1 then you probably won't have anything to update. You should still visit Windows Update just in case (and make sure you have Automatic Updates turned on, too!)