I have the Yamaha rx-v620 and it has really good sound. It has been replaced by the rx-v630. It runs around $499, but can be found on eBay for around $250 used.
I'll throw in a vote for the H/K AVR-520. I think jandr.com and onecall.com still have it for $500. You'd be hard pressed to beat it for clean 5.1 power at $500.