My AMD64 is very happy with FreeBSD/amd64 6.0-STABLE....
It compiles like there's no tommorow...

The only thing missing right now is a FreeBSD/amd64 NVidia driver

It also runs Windows XP 64bit but drivers for the soundcard are still beta so i'm only able to use the front 2 speakers.. Also a lot of hardware I tried, bluetooth, wifi doesn't want to work.. no drivers

That's the catch if you want the latest and the greatest.. Not all hardware is supported (yet) and software isn't taking full advantage of the hardware.. But that's just a matter of time