Go for the 5660 if you can get it at a good price, you'll take advantage of the extra power because they tend to run a little quiet. check out onkyo tx-sr601 if you get the chance. not as warm sounding(notice only on some types of music) but you get better bang for the buck (check out features, also more power and higher higher sampling rate on the processor).
If you can I would avoid the lower-end Yamaha systems. I recommend taking a look at the new Harman/Kardon AVR-130. It will deliver all the power you will need and alot of nice features. Also, the Yamaha recievers you are looking at only deliver about 35W of true-power. The 75W and 80W they are saying they deliver are a 1KHz, and are NOT true ratings. HK rates there recievers correctly and I would look at them. They are a great reciever and VERY easy to use, much easier than Yamaha.