As said above, each person reacts to cold climates in a slightly different way. Some find it difficult to get warm even at moderate temperatures in the negative scale, others find they can sleep warm even at very low temperatures, your body's metabolic rate has a lot to do with this, as well as your general fitness and circulation.
In the title of your post, you've put 'Arctic' and just to clarify is that what you actually want?, or is it cold winter kit, as this also a big factor to take into consideration.
The advice above about using a foam mattress under your down mat is a very good one, as this help reduce punctures and the fact that a very low temperatures the air inside you mat will be less dense (over inflation can also be a problem, as when your body warms the mat the trapped air will expand, and can cause in some rare circumstances seams to rupture, although it is a rare occurrence) the addition mat helps by reducing the temperature range your down mat is subjected to above and below.
at sub -10 the air tends to contain less moisture than at temperatures between +5 - -10 so down bags with a good full 700+ will provide a very good level of insulation per weight.
however this insulation per weight may not always be an issue, if for instance you are travelling by car /snow mobile/ etc, modern artificial fills provide a much better thermal barrier but are more bulky and weighty. this is the choice than you need to make first. synthetic vs. down.
A bag should keep you at a comfortable temperature, as a too hot bag can be as useless as a too cold one, sweating in low sub-zero temperatures can lead to a rapid temperature loss, as the condensation acts as a conduit for heat transfer away from your body, and also a path for the cold in the outer layer to find its way back. The best method is really to trial different bags and ratings to find the best to suit you, however I might add that this should not be done while your actual in the Arctic, as finding yourself with the wrong bag 1000 miles from home is not a funny situation to be in.
Ask friends if you can borrow bags and spend cold nights out in the garden first, to help gauge the rating that best serves you.
hope this helps
Da
