[Gamester17]

This thread is really going off-topic as you are all discussing building custom and niche HTPC boxes (where there already are hundreds of other threads about).

This thread was really started to discuss building a set-top-box style box in bulk for the masses which XBMC could be pre-installed onto and sold in at retail, sure it could use some standard computer parts, but you can not expect it to meet all your dreams and desires for the price that you can sell such commercial boxes for.

You have to apply The Law of Diminishing Return "the majority of the effort should be invested in implementing features which have the most benefit and widest general usage by the potential users".

So forget about HDMI 1.3 and audio over HDMI, I am sure that the majority of potential buyers would be happy with DVI out (with a DVI to HDMI adapter included) and SP/DIF optical out for audio.

Now back on topic:
iBase MI940 looks a motherboard that you has the potential of building a powerful enough system for 1080p is the price is right and since it is a Mini-ITX board it could easily fit a set-top-box:
http://www.ibase.com.tw/mi940.htm