Does the DD5.1 let you select which digital input to use? Are you sure it doesn't disable one if the other is connected? It only has stereo analog input so there isn't any multi-channel gaming.
When trying to get the aiw radeon working with a via mb I read that Cinemaster, the maker of ATI's player, was coming out with a 5.1 compatable player. Their new software doesn't mention anything but s/pdif pass-through though.
If you want full hardware support you'll need to use ATI's player. Which only supports stereo through computer's audio or s/pdif passthrough to a external decoder for ac3 or dts.
The Panasonic SH-AC500 does all that you want, . They're hard to find now but a lot of receivers have external decoder support.(6 rca's in and 6 rca's out)