InsideHW.com has posted a comparison review between Windows XP SP3 and SP2

Microsoft came out with latest service pack for XP, four years after SP2 was released. XP itself is very stabile system with SP2 installed, so SP3 is considered as not so innovative like it was SP2. The essence of this service pack is to bring all patches for XP that are released until now. Beside patches attended for safety of your OS, SP3 also contains few new additions that should provide better network collaboration, easier administration etc.

Windows XP SP3 vs. SP2