+1 for not needing the i7. Thing to remember about HD, is your PC is just playing it back to you, not creating in the first place, as it would be with a high def game. The AMD should be more than enough with a half decent GPU*. Or you could treat yourself to an upgrade for your gaming pc, and use the old bits of that for the HTPC
*And only half decent. Really doesn't need to be a GTX295 or anything!