The bag I use is rated down to 0 degrees. Try to find a bag with a rating at least 15 degrees lower than what you will actually expect as minimum temps. Also, if you haven't tried a mummy bag before see if you can borrow one and try sleeping in it once. Personally, I simply cannot sleep in those things. I'm a big guy, a side sleeper, and I tend to roll over often during the night. A mummy bag ties me in knots. Yes, mine is not the lightest bag around, but a good nights sleep is important than an extra pound to me, so those requirements come first for me, then weight.
Campmor.com has a wide range of choices and some good quality gear at reasonable prices.