Would an i5 not suffice?
I read on Tom's Hardware that the performance of some i5 processors rival with the i7 while costing a lot less. In that way you more money to spend on Netflix (or whichever online/offline distribution system you prefer).
If you plan to use this system for not only for decoding video but also for a lot of encoding video, get an Intel. My daily PC is an AMD and I am fond of using it. Until I want to encode some video, there is where Intel always had the upper hand in my experience. However if encoding is not a primary concern, get AMD...which leaves you again with more money to spend on content to watch.
Onboard audio quality from motherboards nowadays is fine enough by my standards. Besides that, spend money on a decent amplifier (
Revox) and good speakers (
Wharfedale is my brand of choice), those pack a whole lot of "punch" in small boxes. Not only your ears will love you for it, your better half will too. Again from personal experience, I have to meet the first woman that likes to have big speakers in any room.
JavaJones is absolutely right about video cards and media center software.
EDIT: Added link