DragonSteelMods posted a performance comparison between Windows XP Service Pack 2 and Service Pack 3. Here's an excerpt:

So for my own purposes I decided to run a bunch of tests to see if there was any real differences, after all Benchmark and testing software is designed to pick up very small imperceptible differences in things that we normally wouldn't notice. We wouldn't notice a few nanoseconds or milliseconds most likely but the software will as that's what it's for, to measure things like that. So I ran SiSoft Sandra, Cinebench, 3dmark06, SuperPi Mod 1.5 and TmpgeNC before and after the upgrade to SP3, I decided instead of just keeping it all to myself, why not share the results, and well there is a difference between SP2 and SP3 and it's a mixed bag of results really, in some cases SP2 is better and in others SP3 is, but there is obviously something going on, something changed and I'm not sure if it was for the better...

In related news, Microsoft has just released a whitepaper, which compares the following features and capabilities of Windows Vista with SP1 & Windows XP with SP3 : Security, Management, Deployment, Mobility, and Productivity.