Agreed one of the comments on that atom board is "handles 720p pretty well." Meaning it doesn't! Any most TVs are 1080p anyway so you're looking at a board that can't handle mkv files too well let alone blu-ray discs.
A 440 (x3 Core) would be overkill, but you could actually use the system as a real computer ... even fairly agressive gaming ... ~$75 The fastest Athlon-II x2 Core (even with only 2GB RAM) will kick the living snot out of any ATOM.
Micro-ATX is your friend and 4200 graphics ROCK for HTPC