Help me determine which sound card is best for home theater system?

I have a Sony Home Theater that comes with a receiver and 5 speakers that I want to use for my desktop computer. Currently I have one of those RCA cables that go from my receiver to my computer's headphone jack. I believe this is only 2.1. Is there a way to get true surround sound of 5.1? I believe I would need to add a sound card as currently it's only an onboard sound card. Could someone recommend me a good sound card that will enable true surround sound?