I get down to about -25 or -30 before I pack it in and walk out <img src="/forums/images/graemlins/smile.gif" alt="" />

Dunno about others experience, but unless I am in a hot tent, I prefer multiple bags myself. In my experience the big hefty down bags will work really well for you the first night, and
then unless you can dry the frost out of them you'll get colder and colder. The problem is that the moisture from you ends up freezing *inside* the bag as you sleep. This wets out the down. You have several options to help with this:

1) Use a synthetic bag - this is big and heavy, but easier.

2) Use a VBL (Vapor Barrier Liner) INSIDE your down bag - this is basically sleeping in a moisture proof sack inside your sleeping bag. This keeps the water out of it. problem is - well, it's sleeping in a baggie - some people love em, some hate em.. I fall in the latter category.

3) Use multiple bags, and go for a hybrid approach. This takes a little more experience but isn't that hard. I sleep in very cold weather with a -10C down bag, *inside* a synthetic sleeping bag or overbag - typically anything from 0C rated exped bag to a "wally world" basic synthetic bag. it doesn't have to be that warm a synthetic bag - it just adds wamth, and most importantly, puts the frost layer (the depth where it is freezing) *outside* your down bag. This means the down doesn't wet out. If you haven't slept outside in such
temperatures before, consider your clothing as well. I sleep in full longjohns, cashmere sweater, often a pair of gloves and for absolutely sure a "bank robber" style balaclava (the kind with only eye and mouth holes) with a fleece toque *overtop* of the balaclava. You don't want to have your face and head so cold you have to keep your face in your sleeping bag (breathing moisture by the gallon into it)

Either way - if you take down make sure you consider frost!

Stove? white gas. don't mess around. Lots of them will do well. MSR whisperlite, or something similar. I don't mess with canister stoves or alcohol in winter when I'm melting snow. (and I never use white gas otherwise either <img src="/forums/images/graemlins/smile.gif" alt="" />