x2 on Bobby Urban's comment.
For down bags, check out Feathered Friends and Marmot. They aren't cheap but they'll keep you warm. I would highly recommend a good bivy if you are going into the back country as well. And if you are planning on sleeping in it, get used to it at home. A different world and you can spend some uncomfortable nights if you aren't familiar with it.
As for temp ratings, I err on the side of warmer than I need. The last thing I want is to have a chill when I sleep. I have used 15 degree bags in 30 degrees and been cold. Again, I err on the side of warmer. Also look for good draft protection. That makes a world of difference. You can also look at sleeping bag liners which will add some warmth at a cheaper price.