The Windows XP version for 64-bit Extended systems [WS2003 codebase] has been in alpha since December 2002, at which point build 1069 was sent to some manufacturers. Since then, a few other versions (up to 1137, I believe) have been released, and also downloadable for MSDN subscribers. Note that Windows Server 2003 beta for 64-bit Extended systems has recently been made available to the public, either by download or you can get the disc; the build is 1069, with 09/06/03 dates (whereas the pre-release WS2003 beta is currently build 1137). Some find they could run better under 1069 than 1137. Target CPUs are Athlon 64, Athlon FX and Opteron running in long mode, and the WOW64 engine will emulate some WIN32 applications. Of course, you can install 32-bit OS with these CPUs, but you'll loose the benefit of memory linearity beyond 3.5GB. The beauty of 64-bit is to accommodate more memory, enabling to run decent RAM Disks, or a separate space to a leaky WIN32 application, etc...
Note that the 4GB DIMMS from Crucial currently cost $6999.99, meaning up to $56,000 of memory for a fully populated (32GB) dual Opteron board (instead of about $2000 for 8GB), and $112,000 for a quad Opteron board (64GB)..
64-bit drivers are slowly coming out, but at least you can multi-boot with some other OS if you need anything exotic..
No one can accuse MS of monopoly as far as 64-bit extended systems
Good luck,
Chri