I'd recommend Mandrake Linux 10.0 for AMD64.
Installs and runs like a champ on the AMD64 mobos. I've got my Chaintech VNF3-250 w/ AMD 64 3200+ set up as a dual boot w/ it and Win XP. I'm a new noob on the Linux but it's pretty sweet. (the XP side for games since it's my fastest machine.)