Well you need a 5.1 sound card, and a 5.1 home theater system.
Get an 1/8th inch to rca adapter and plug it into the second output on your sound card. Then take and connect the rca cables from the adapter to the input on your stereo system. That should be it. Thats the way I have it set up and it works great. Make sure your audio settings are set to 5.1 also.
If you have an spdif output on your soundcard, you could use that as well. Your stereo system has to have an spdif input though.
Here is an adapter like the one that I am using. It has male connectors on the rca side, but you can get female ones I think. If you get one like that you just have to get a couple barrel connectors to connect the rca cables up, no big deal. This one was actually given to me so I decided to just use what I had.