Windows XP x64 is my favorite version of Windows. It really depends on the software and hardware you are using. If all of your hardware has x64 drivers... especially your printer (mine does from Canon)... I would say go for it. Also, Make sure the software you use works with it. In x64, Microsoft dropped all 16bit support, which means no more legacy network protocols, or DOS support for programs. Other than that, I love it. Yo will start to see more and more applications and games get 64bit variants. For example, Half Life 2 just came out in 64bit last week.
However, if you don't want to deal with application and hardware compatibility, just get Windows XP Pro or Home. And remember, this time next year Vista should be out, so you won't be missing out on much.