If you are just going to play movies you don't need much. The Lian Li case that I mention above uses passive cooling (no case fans) and you cannot hear any noise at all. Warm air is exhausted by the PSU (Seasonic G360) and I'm running the Intel stock cooler on an i3 (you can get by with a Pentium if money is an issue). CPU runs about 3C above ambient room temperature and I use on board Intel HD4000 graphics which are perfect for HTPC.
Helps to have measurements.
