Lots of good stuff with that budget. Anything in this range will work fine with your setup now, and if you move to better front end you'll definitely have a huge improvement.
Yamaha V663 - Prob the #1 ~$500 or so receiver.
Onkyo 605 $429 on sale now on par with the V663, just about 8 months old at this point.
Harmon Kardon 247
Sounds great, seem to break more than average. Decent deal, but does NOT do the lossless audio decoding like DTS-MA, Dolby TrueHD, etc..
However, if you can put up with a silver unit, and can swing $699 I would get the Onkyo 805S (silver). I have the black one, and paid more than $800 for mine, no one really wants a silver unit these days, so they're priced down A LOT.
What you'll get over all of these? Great Burr Brown D/A converters, HUGE power amps, (130wx7), all the THX certification you could ever need. It'* built with all of the major pieces seperated, Pre-amp, amplifiers, powersupply are all in their own area, not on the same boards. The only real issue that'* been common with them, is the fact they seem to have a bit of audio lag in them no matter where you set the A/V sync. I would go this route if you plan on moving to a nicer speaker setup than what you have later on.
This is essentially the knockout Onkyo put out last year, it'* MSRP is something like $999, and most $1500 receivers can't touch it, the nearest ones with the same THX certs are usually $2k. They really blew the market for higher end receivers wide open with it.