The idea is that if two teams played today, the one with the higher ranking would win a best of 7 series. That's pretty much what power rankings are. The Habs started pretty low, and probably reasonably due to the lack of momentum from the preseason, but they should keep moving up.
Also, SI.com's power rankings are much more "sticky" than most others, so teams generally don't move up many slots in one week unless they handily beat a much higher team. Notice that only one or two teams moved more than 3 spots.
Habs at 20th is little low, they should make the better half considering they got OT point from the top team in these rankings. I see couple teams overrated in those rankings - Nashville hanging there at 7th place and Hurricanes at 13th seems a bit much right now.