The final SP3 release is version 5.1 build number 5512 (5.1.2600.5512) confirmed by Chris Keroack and the general public will be able to officially download it starting from April 29. The installer package from Microsoft is ~580MB.

MSDN and TechNet will only get access along with general public, not before them which caused a lot of complaints and uproar yesterday.
Quote Originally Posted by Biker View Post
WindowsXP-KB936929-SP3-x86-ENU.exe
size 331,805,736 bytes
That's the much older one AFAIK, build 3180 RC1.