Octavius,
Actually the x86 64 bit version of Windows XP should be out later this year. 3rd quarter probably.
I'd have no concerns with AH, but I do have concerns about initial driver availability as the drivers for your hardware will need to be 64 bit. The move to 64 bit is now inevitable really, as Intel is also going to be releasing x86 64 bit chips. Because of that the driver situation will get better pretty quickly, but older hardware or hardware from companies that have folded (my SCSI RAID card) are probably out of luck.