Windows XP 64 bit edition was based on Server 2003, as it was released after it. You can even say it was 64 bit, client edition of server 2003.
Windows XP service packs were based on, well, Windows XP. Because that's what service packs were. Lot s of new features have been developed for SP2 (and possibly some backporting from 2003 happened, too) but XP is still XP.
Windows XP service packs were based on, well, Windows XP. Because that's what service packs were. Lot s of new features have been developed for SP2 (and possibly some backporting from 2003 happened, too) but XP is still XP.