2003 Server is supposed to run faster than XP but it's only because a lot of features are disabled. For example, the Windows Audio service is disabled by default, which means no sound. DirectX isn't installed by default either, which means no games.
Of course, you can install/enable all of this, but then that just goes on to make it XP, and u won't notice the speed difference. Besides, 2003 is a server OS, and will cost a LOT more compared to XP.
I say, just stick to a licensed version of XP, unless you're planning to use the server features of 2003 Server.