That external exitgy box is one of the very very very few boxes (that aren't full sized receivers) that will take 5.1 over optical/coaxial. The point of that thing was to use your cheap 5.1 pc speaker setup with consoles and dvd players.Things like that just aren't made anymore which makes that external exitgy sound card so damn rare, I know this because a few years ago I was looking for it to use instead of a receiver and to use my 360 with some cheap logitech speakers for 5.1 gaming.
Also, I'm not entirely sure you can just plug a 360/PS3 to the optical input on a soundcard and expect it to give you a 5.1 signal, it'll instead be knocked down to 2.0/1. This is me just going off of memory when I wanted to do this before and found out the only thing that could do it was that external extigy. This was about 6-7 years ago so things might have changed.