It would be hard to limit it to 5.1 as most are now 7.1. However, those last 2 channels can sit dormant - until you decide to use them at some point in the future.
As far as recommending a receiver in this price range, I'd point out that the Yamaha HTR-5790, purchased through an Internet vendor for $530 or has to be about the best deal.
Aside from that, you could look at used gear. Since your desire is for a 5.1 system, that leaves lots of options open. Dolby Pro Logic IIx is not a requirement and the former, DPL II, would suffice.
I'd look at the not yet released Denon AVR-1905 at $499. It's feature list is pretty impressive at this price range. Most notably powered zone 2 out, 3 component video inputs, and component video up conversion.
Depending on what you have for speakers, the Panasonic XR-25, XR-45 or newer XR-50 and soon to be released XR-70 digital amp receivers are considered by many (on many different forums not just this one) to be the best quality sound for the money. All under your $500 target (not sure about the XR-70).
Reports from numerous users who are using the XR-45 as a 2-channel integrated amp to replace >$5,000 traditional preamp/amp systems to drive high-end speakers testify to good sound quality. This is based on using the digital output from a CD or DVD player to the digital input of the Panasonic receiver.
These Panasonic receivers all use the same TI (Texas Instruments) digital amp technology.
I think the Kenwood (7100?) is also using this same TI technology.
I'd look at the Pioneer VSX-1014TX. It's exactly the same as the Pioneer Elite 52TX which sells for $800. It's available from www.onecall.com w/ free 3day fedex shipping for $450. I got mine from a local HT shop for $450 out the door (including tax). Mine will arrive on friday or monday so I should be able to give a better review and comparisson to my current Elite 43TX then.
I would suggest the Sony DA1000ES for that price. Though Sony might not be the brand with the most street credibility here, it works and has got good reviews in europe.