s/pdif = coax digital.
You would have to use a mini plug to RCA plug adapter to make it work on a coax input for a regular receiver. Idealy you should fine a mono adapter, but I use a stereo "Y" splitter adapter and when I plug the left (white) side into the Coax input it works just fine.
The bad/good part is that this is still part of your sound card. If you have a crappy sound card it will still be crappy even through the digital line. If your lucky though maybe the part of the sound card that is causing the crappy sound isn't used in the digital line, but I wouldn't count on it.
If you want a better sound card you can get a Sound Blaster Audigy or a Turtle Beach Santa Cruz for fairly reasonable price and they sound great.