As the others have said, speakers matter more. They are vastly more important. Any receiver from a respectable brand will work okay with speakers of normal impedance and sensitivity, so unless the speakers you select are difficult to drive, you can go cheap on the receiver and still get good sound.
Also, speakers are, by far, the better investment for the future. This is because they are always coming out with new features for receivers, that people feel the need to get. So the receiver is basically going to be replaced eventually anyway, so putting extra money there is a waste. But you can still use old speakers, and if they are great, then there is no need to replace them. And speakers also can last a lifetime, if they are not abused and if they are well-made.
With a budget of $600, I would personally go with a 2.0 channel system, getting the best pair of speakers I could find. Probably, I would go with bookshelf speakers, and simply sacrifice deep bass for better sound for the higher frequencies (and this, by the way, is coming from a person who owns a pair of SVS Ultra subwoofers; I love deep bass, but not at the cost of unsatisfactory sound from the midbass on up).