How Can I Connect Pro Studio Speakers to a PC?

I know others have done it, but I recently bought a set of Cerwin Vega speakers (no cables or interfaces yet) on sale for Black Friday and I'm trying to find the best way to hook them up to my PC. I currently have a Creative SoundBlaster X-Fi Xtreme Audio card that has optical input and optical output along with the usual connectors but I'm fairly certain I need a different interface in order to get true 5.1 surround sound from the speakers. What options are out there? Either a device that accepts optical coming in or something that's USB/PCI that lets me plug in RCA outputs would be great. I have a regular desktop PC with plenty of room inside for additional components. It also has USB 3.0 if there's a scant few audio receivers that take that as an input, heh.

Edit: I also have the Nvidia Geforce GTX 460, which has HDMI output. Can I use that to output plain audio to an interface?

Not quite what I meant, heh, though that would technically work for a basic system. Apparently there's a difference between speaker wires and RCA cables. I ended up going to Best Buy and found out that optical is actually fairly common nowadays, whereas five years ago it wasn't heavily used. He helped me pick out the right basic AV device I needed in order to hook everything up. Now to enlist dad's help in hooking it all together. ;-)

The Denon AV device that I got works like a charm! It has all the hookups I could ever need. Dad helped me put it together until 3am. I can't wait to test this at full volume. ;)