Seems likely. Someone who has seen the actual source would have to confirm, but my understanding from back in “the day” was that Server 2003 was based on the XP SP0 tree, but that XP SP2 and later were re-based on the Server 2003 tree as part of the security focus of SP2.
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.
