Your graphics card has sound outputs? Are you sure? Also, if it only has one red and one white, you ain't gonna be able to get sorround sound out of them, only stereo. What sort of digital output are we talking about? None of the systems you've specced had a decoder, so digital sound out wouldn't work for sorround sound either, unless you get something with a decoder in it.
EDIT: If you're interested, I may have a pair of these for sale:
http://www.videologic.com/Products/P...PageType=Intro
Which retailed for £140, and I may sell them for £80ish, not sure yet, as I need to check if they are still working when I replace the fuse in there.