I'm not familiar with exact specs, but you should expect to pay around $500 for an HDsound capable system (just the reciever). What I do know is optical carries 5.1 sound, but not uncompressed PCM. HDMI (or direct player output) is required for PCM sound. I'm not sure what Dolby HD needs. My brother got an HDMI Onkyo reciever (sound processing, unlike some so-called HDMI recievers) for about $500 and it's pretty decent. I would say spend the money on something like that, unless you buy new recievers every other year.
Upon further investigation: check this out:
http://en.wikipedia.org/wiki/Dolby_TrueHD
Seems the old optical/coax connector is not sufficient. Look into getting an HDMI sound processing unit. Sub-$500 systems usually switch HDMI, but may not process it.