I'm in a similar boat as the others. I'm looking to upgrade my receiver and have been reading the various posts here. The Denon 2808ci seems like the best deal at the moment but I thought I would mention my speakers just in case. I have the Paradigm Monitor 11's in the front and the Paradigm ADP390's in the rear. The specs are posted on the Paradigm's website, since I'm a newbie it won't let me post the direct URL

My center channel is the odd man out, it's a JBL SC305, here is what I found for the specs on that:
High Frequency Dome Transducer 14mm Titanium Laminate
Low Frequency (High Polymer Laminate) 5"(2)
Crossover Frequency 2.7 kHz
Frequency Response (- 6 dB) 100 Hz to20 kHz
Sensitivity(1 Watt/1 meter) 87 dB
Nominal Impedance 8 ohms
Recommended Amplifier Power* 10 to 100 Watts
I'm looking to spend 500-600 as well. I'm open to any suggestions or opinions.
Thanks in advance