The only reason you would need 5.1 is if you think you would want home theater or 5 channel audio via SACD or DVD-A. I personally like Onkyo much better than Yamaha.

If you decide to stick with 2 channel, I'd suggest checking to see how much a British integrated would cost you from Creek, Arcam or Roksan. You will gain far superior stereo sound from one of those over a receiver. What ever you buy, be sure it has a phono stage for your turntable. If you have to buy an outboard phono stage the Creek OBH-8 is unbeatable at it's price in my book. In fact, I had to spend quite a bit more to improve on sound when I upgraded from mine.