I built a HTPC back in April and I used the AMD A6 3500 apu specifically for the 65W triple-core. It runs cooler and draws less power. I also replaced the stock noisy CPU fan with Scythe SCSK-1100 CPU cooler. The thing is silent! I also used 4 Enermax UC-8EB 80mm case fans just make sure the temps stayed down. This box is whisper quiet!
As far as noise goes, besides the CPU cooler, your main culprit will be your Power supply fan. Since you already have it you should know what it sounds like. Personally I think that PSU is a little overboard but if you have it and its not to loud, save the money an use it.
I run HDMI audio from my HTPC(Gigabyte GA-A75M-D2H motherboard) to my Onkyo AVR and run 5.1 no problem. I dont have 7.1 setup and probably never will in this room so I cant answer to the 7.1 question. I use XBMC on my HTPC and stream from my WHS 2003 box. For a controller I use the Iogear GKM561R Wireless Keyboard. I like this a lot, it has a trackball mouse built in and works very well. I just wish the keys were backlit.