For HD video playback using integrated graphics you really need one of 3 chipsets, 780G (AMD), 8200 (nVidea for AMD procs) and G43/5 (Intel). As to sound over HDMI the 780G chipset supports it natively I think the other two have work arounds to make it work but will depend on the board.
I was under the impression that if you went into S3 (sleep or suspend whatever it is) that you could get it to wake up for recordings.
For a low power media centre PC I would be looking at AMD the 4850e for its 45W TDP will be plenty enough grunt and will stay nice and cool