Or you could get a motherboard with an integrated gpu and get everything a bit cheaper.
The amd 780G is really something when it comes to decoding blu-ray movies, and coupled with an energy efficient(maybe even passively cooled) processor and powersupply you get a pretty awesome HTPC.
You also get sound over hdmi, so I'd say your best bet would be to spend a little less on your HTPC and invest in a good receiver and sound system.
Here's a review of it:
Here's a htpc I built pretty quickly(newegg.com):
And I know you could get it for less if you switched out some components like the hard drive and memory for something cheaper.