There is an AMD 64 specific build for 710, but it was quirky at times. Do some searching if you're really curious. I started with the 64bit, then reinstalled the 32bit on my dual core AMD 64 based server and have been happy. No idea if a 64bit version of 810 will be out, but since there is very little performance difference with LinuxMCE it shouldn't really matter.
Not sure if you realize it or not, you can install and run 32bit linux on an AMD Phenom 2 just fine.
As nite_man suggests, check out the nVidia ION based systems for your media directors (the HTPC client at your TVs). The only drawback at the moment is they apparently are not powerful enough for the new HULU integration (flash based). However, things look promising for this as well: http://www.nvidia.com/object/io_1254722910215.html