Audio speaker/decoder/card selection

[supernoob]

currently I'm just using the onboard sound and my 2 speaker stereo to provide sound from my htpc. I'd like something a little more.....Home theatre'ish.

unfortunatly I'm clueless when it comes to what I need. I tried looking around but I keep seeing the words reciever and decoder being thrown around and I'm not sure what they do or what speakers they come with.

Could somebody help me find a solution for under $250? I doesn't have to be top of the line, just something more substantial than I have now that I can be proud of when people come over.

A 5.1 setup would be just fine, maybe like the Creative Inspire GD580? dunno if I need a card or not either. here's the onboard I have

Audio
• AC'97 link controller integrated in VIA VT8237.
• RealTek ALC655 6-channel audio.
- Compliance with AC'97 v2.3 spec.
- Meet PC2001 audio performance requirement.

Thanks for the help everyone!