There was a 64 bit Version of Windows XP available, but I doubt you can still buy it while Microsoft is trying to push Vista in every new computer.

edit:
Whoopa! Surprise!
Just look what I found at reichelt.de
http://www.reichelt.de/?;ACTION=3;LA=2;GROUP=N11;GROUPID=3913;ARTICLE=58895;START=0;SORT=preis;OFFSET=16;SID=29wkxdOqwQAR0AAGt06Qs704e80ab6a9ce777989a45f0a70f9647