Of course there are 64 bit versions... The Unix OS has been 64 bit for.... Let me put it this way; In 1999, FreeBSD was already usable as a 64 bit OS.

Anyway, I have a 64 bit machine now too, and I have these:

FreeBSD 64 bit (IA64 / AMD64)
Slackware has "Slamd" and also a 64 bit port
SUSE and Open SUSE ALL have complete 64 bit support
Red Hat and Fedora as well have it
Debian also has 64 bit
There are more, like Mandriva and so on, but it would take a long time to list them all, so whatever distro you like, probably already has 64 bit. I don't know of even ONE distro that doesn't come in a 64 bit version.