Windows XP SP3 is for x86 editions of Windows XP only. The x64 editions of Windows XP were serviced by Windows Server 2003 SP2. For additional information, go to Windows Server 2003 Service Pack 2.
SP3 includes all fixes since SP2 was released in August 2004. XP Service Pack 3 is around 80MB (typically 66.9 MB) through Windows Update (316MB direct download). SP3 needs SP1 installed if you are patching an existing installation (ie. you cannot patch XP RTM to SP3).
Windows Update download took under 10 minutes for me this morning after which verifying files took another 7 minutes. But then the installation failed with the following
That means install encountered ERROR_ACCESS_DENIED in some operation it was attempting. I am running as Administrator so I am not sure what access is needed. WindowsUpdate.log had the following line giving more details
2008-05-07 10:56:45:480 1040 46c DnldMgr FATAL: DM:CAgentDownloadManager::GenerateAllDownloadRequests: CACDS: SusMoveDirectoryContents failed with 0x80070005.
I will probably have to try the network download and see how that goes. Microsoft may have some glitches to address still.