XP SP3 is just SP2 with updates that was out after SP2, and some new things to make better work networking with a Vista machine. If you already optimized XP SP2 fully updated for performance (disable all the stuff you don't use), you won't see any performance increase with SP3. XP SP3 is like Win2k "SP5" (was it out?! anyway). Nothing else, nothing more.
On the contrary - there's some new stuff in there too, some people see ~10% gain, although it's quite rare.
I installed it on my Virtual PC WinXP as a must as I develop my software which I need to make sure it run smooth (I run Vista 64), and I see no difference at all.
I tried to install it yesterday, but it wouldn't download. Don't know why, maybe the Windows Update site was broken... Andy