Our own Cameron Johnson just finished completed some basic testing comparing the performance differences between Windows XP Service Pack 2.
We used the leaked and non-final Service Pack 3 (Build 3180), which made its way onto the world wide web last week after being leaked out by MS BETA testers.
We tested using the latest versions of 3DMark05, 3DMark06 and PCMark05 and saw vertically no performance increases by updating to SP3 from SP2. We thought there might be some slight chance for somewhat of a performance boost but everything was exactly the same (or within the usual margin of error, which we always see).
We would have loved to report that SP3 provided a performance boost but at this stage it doesn't appear as if you should be holding onto Windows XP in the hopes that the upcoming service pack will boost your PC performance.
We'll keep you posted on further SP3 updates as they come in!