Home theater for a poor college student

I'm trying to put together a home theater system but on a tight budget. I already purchased 5 speakers, all polks, RT5's in front, M3's in the rear and a CS-175 for the center. I know I probably should've bought a better brand, but being a full-time student the price was right (I found most of it for very little money used) and the sound was great for my fairly small room. I'm ready to upgrade my receiver but I'm a little clueless on what to buy. I've been looking at the intro models of HK and Denon, but I'm not sure what would be right for me. If possible I would like to find a used item because my max price is probably $500, but I'm hoping to find something for less. Any suggestions would be great!