There's a little-known version of XP that is true 64-bit (regular XP was 32-bit). The 64-bit version will address much more than the ~3GB max of regular XP -- 128GB. And it supports dual CPUs (not just multiple cores, but multiple distinct processor sockets).
http://en.wikipedia.org/wiki/Windows_XP_Professional_x64_Edition
Unfortunately, it played second fiddle to the failed Vista release, and never got much attention.