Marmot makes a great line of sleeping bags that go all the way down to -40. Going with a synthetic bag is much cheaper, but the 4-season ones are quite heavy and bulky. If you go with a down bag, try to get one that has treated down, so that it will stay loftier if it gets a little damp.
It will run you about $500, but the Mountain Hardware Phantom 0 degree bag is super lightweight and efficient. If you're not willing to go that expensive, the Kelty Cosmic will do you well.
You can probably get by with a zero degree bag, if you have a good tent or are sleeping in a snow cave. Bring extra layers or a down jacket if you are worried that it won't be enough. You can also add a liner to a bag to make it warmer. Worst case scenario, you can take a summer bag and layer it with a 3-season bag. That might be heavy, but it will be cheap and toasty warm.
I manage a camping and fishing store, as well as go out on my own expeditions. Hope that helps.