I just installed it for a friend on his amd64 3500+.
Man, it rocks... I compiled Xmame in 15mn. It takes 40mn on my 32bit 2800+ and 20mn on my 4 PC cluster. All the system is very reactive. Of course, installing 64bit programs can be tricky, but http://linuxpackages.net/
provides 64bit slackware packages.
You can install a 32-bit without problem... but if you want to dig Linux, then a Slackware-based-64bit-distro is just perfect.