Aha, carrying all in rucksack for me points on one thing: weight! Although Norway is one of the countries in the world that uses most electric energy there are no outlets in the mountains so long battery life is essential and the Montana has too short battery life and weighs too much. It is a good solution for MC (where you have access to power) or mountainbike as long as your tours are under say 4 hours. And the 276C is excellent on a MC but it is a heavy lump and should not be carried...
So you should go for an etrex or gpsmap60/64 in my opinion. If you are counting grams, use lithium AA batteries that is really lightweight and lots of energy even when it is -25 degrees (winter climbing for the hard ones...)
Good topo maps for Norway can be found here
[Only registered and activated users can see links. Click Here To Register...] or use the sticky index to find what you need.