The DIY community if full of different versions of these but after some research this is in my opinion the best design.
One of the old controller ports is converted to USB so the old controllers can be used! You´ll need a chip from http://www.retrousb.com/. To map the controllerkeys to stuff i use Eventghost (A for Enter, B for Backspace, Start to launch XBMC etc)
What comes to hardware i guess any Mini ITX board will do but i picked the Asus AT3IONT-I DELUXE. This is why:
- Laptop style power supply
- Comes with a remote. Not perfect but does the job. Remapped keys again with Eventghost.
- Plays all HD videos without problems
- Good price
UPDATE: I added som USB ports under the lid. Pics coming later...