I'm currently running a HTPC based on a AMD Athlon 64 X2 4850e on a 780G based motherboard. All media is stored on a NAS. This setup works great for 720p, but it's not as silent as I'd like.
I've seen a few postings about XBMC on BeagleBoard, but it doesn't seem to be in a functional state as of yet.
What I'd really like is to have a passively cooled box that's able to play 1080p H.264.