I have a similar setup and am running Suse 10 with great results.
I have also installed (as a dual boot) Slamd64 (slackware) on a spare partition that I am using to test different distros. It was slightly harder to get going but in the end works great also.
I got frustrated installing Gentoo (but I am impatient) - I will try again another day.
I did boot up the live Ubuntu for 64bit cpu's - and it was fine for those few minutes.
The live Kororaa-xgl-demo was very eye opening
- it is my next trial of Gentoo that I will install (Kororaa without the xgl as the live cd has no installer).
Everything depends on your preferences, I feel that for overall ease of use Suse or Ubuntu will be satisfactory, but more "hands on" installation distros can be very fulfilling.
Other distros with 64bit ports are Fedora and Debian. I am sure there are others - if you can, download a couple and try them - or just order an Ubuntu cd for free
Hope this helps,