Have most packages been ported to the amd64 architecture, or is there
a limited selection?
Check out http://buildd.debian.org/stats/
Yes, most packages have been ported.
I found the amd64 "testing" version here:
http://amd64.debian.net/debian/dists/testing/main/installer-amd64/beta2/images/
But I wasn't sure if most packages had been ported to use the 64-bit
OS. Any problems there?
AMD64 is now an official Debian port. At least unstable should be as
usable as any other port. AFAIK testing is still not completely
built.