A new receiver is the way to go - it is possible to buy separate decoders, but only in the audiophile domain, and that doesn't sound like what you are looking for.

What would your budget be? You can pick up some great receivers for under $350 now... eg Onkyo TX-DS595, http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1353819558
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