I believe XP64 was OEM only, there was no retail availability. I had.. uh ... obtained a copy and installed it on my brothers gaming PC and it ran surprisingly well for an OS that barely made it to market.
XP x64 was available in retail, I used it for a pretty long time. The biggest issue was lack of drivers for stuff like printers and scanners, though iirc later on the Vista 64-bit drivers would usually work.