If you plan to go to the arctic I suggest you buy a sleeping bag system that goes down to at least -30. Depending on which time of the year ofcourse, in winter it wouldn't be strange if it was -40 C.
I myself have the Alpkit SH 1000 and the SH 400. The SH 1000 goes down to -16. This winter in arctic Sweden I slept in it at around -18 and it was still very warm, I had no bivy bag, no inner liner, just 1 pair of light woolen underwear (around 100-200gr) and was sleeping in a small tent (about 5 degrees warmer then outside). Then with the SH400 inside the SH1000 I slept many nights at -30C and was very warm, I wouldn't be surprised if it would work comfortably down to -40C as I hadn't even closed the bags properly (too warm!). That for a total weight of 2700gr for the bags combined.
A great advantage is that I can use the SH400 in the summer as well. So with these two bags I need nothing else the whole year through. Although I take a nanok with syntethic filling on canoe trips in summer, as it's cheap and dries fast. Though the 17 day trip I just finished I had my sh1000 as it could get quite cold in the nights, it was north of Östersund, there was still snow lying besides the roads when we got there, winter ends in the beginning of May.
For a lighter system I suggest you buy the Pipedream 800 and 400.
If you only expect temperatures down to -25C you can easily use the PD800 or SH1000 together with a bivy bag (3-5 degrees extra), inner liner (3-5 extra) and some extra underwear (...). But the bivy and liner might weigh a kilo together as well...
