A little searching shows that Fedora, Debian, Gentoo and SuSE all have x86_64bit support.
SuSE has a nice install/configuration utility and bundles a lot of software, while Gentoo compiles everything from source for lengthy installs but optimum performance. Fedora is like Red Hat, and I've read someone say they've heard that Debian needs “tuning”.
Or build your own
If you have an AMD Opteron processor (rather than the Intel range that don't support 32-bit code very well at all) then you can get most of the benefits of 64-bit just by installing Slackware and recompiling the kernel (and any other critical apps you use a lot) for your processor.