To run 64bit programs you need both a 64bit processor (e.g. Athlon64, Phenom, late model P4s with 64bit extensions, Core2) and a 64bit operating system (e.g. Windows XP Pro 64bit, Windows Vista 64bit, Ubuntu Linux 64bit, Solaris 10, OpenBSD 4.2 64bit, FreeBSD 7.0 64bit... there are dozens of them). It isn't really worth the trouble unless you need more than 4GB of RAM, as you run into some weird problems, e.g. if you use the 64bit version of Firefox, you can't install the Java runtime or Flash player as they only support 32bit browsers. There are ways around this, but it's a pain. Even though I have a 64bit processor (Core2 Duo), I run 32bit operating systems instead because it's less hassle. There is actually very little extra performance to be gained by moving to 64bit except for a few applications that are optimised for it.
[email protected] seems to like it, if you're running a 64bit SSE3-optimised client, if that's your thing.