Whats your budget like?
IMHO, If you have analog outs available, 5.1 computer speakers like Logitech Z-5500 may be the way to to. This specific Logitech model can do DD and DTS decoding via Optic or Coax inputs. I cannot comment on if it will give you the volume to fill the room. Look into Klipsch and Creative for alternatives.
If you need DD, DTS, DVD-A, SACD decoding and dont want to go with computer speakers, you will have to go HTIB. There are some pretty decent option from Denon and Onkyo but they run around $600 - $800. A decent HTIB should easily best any computer speaker 5.1 setup.