This is an on-going post and is far from complete. I hope that through input from other users it will grow, becoming ever-more comprehensive.
My goal is firstly to create a central media server for my home that can serve up media to TVs and computers. I'm tired of digging through spindles of DVDs strewn around the house to find a movie to watch and then conceding that I can't watch it in the bedroom because the DVD player in there that can't play media files and I can't watch it in the living room because someone is using the computer. No! I want a media player for both TVs and media server to supply them. And as a plus, Amahi's modular software will let me add other cool capabilities from which I can get some utility. Of course all of this is just academic until I can make it work, so on that note lets get going!
Here's my original concept (on graph paper of course).

Currently my iMac sits on the coffee table and is connected to the HDTV, as a screen-spanned external monitor, via Displayport-HDMI cable and the surround sound decoder via optical cable. It has a 500GB external drive that holds a selection of movies that I use VLC to playback. It works well but is limiting, especially in storage. Instead I want to use the iMac to manage the media system, since the HDA will be headless, and hand over playback to a dedicated box.
I am considering using the HDA as a full HTPC to reduce the need for a separate playback box in the living room, but only if that is technically feasible, and since I like to think about long-term flexibility the hardware I selected for this project is fully capable of playing the role of an HTPC. What do you think of that?
Well that's a good start for now, I'll cover hardware selection in my next post. Did I mention this is my first PC build?