A: Use the Windows Server 2003 SP1 DDK -- Use the Windows Server 2003 x64 Build Environment

As of early February, this kit was still in Beta testing, and was available either as part of the beta program or via MSDN.  But, that's not exactly a world-stopping problem because neither of these operating systems were released yet either.

Why, you ask, would one use the Windows Server 2003 X64 Build Environment from the Server 2003 SP1 DDK to build a driver for Windows XP running on the x64?  Well, basically, because it's the only x64 build environment that's supported.  There isn't one any older kit, and there isn't an x64 build environment in the kit for XP.  So, the Server 2003 SP1 Build Environment, in the Server 2003 DDK is the one to use.

Just to make sure that we're all on the same page: No, you can't use the x86 build environment (this is the "regular" build environment in the DDKs, the one without other qualifiers) to build drivers for the AMD-64 or EM64T (because drivers for x64 systems need to be 64-bit code).  And, no, you can't use the IA64 build environment either (the IA64 is a 64-bit machine, but it's a different instruction set).

I hope that helps.