Sadly it is not trivial to do. See Geoff Chappell's article:
http://geoffchappell.com/notes/windows/license/memory.htm
You'd be better off with Server 2003 Enterprise, which is also supported until July 2015 unlike XP.
"The 32-bit editions of Windows Vista and Windows 7 all contain code for using physical memory above 4GB. Microsoft just doesn’t license you to use that code."
A huge pity he no longer blogs.