Jim is so right--make sure you don't sweat while you're moving! When you stop, and it's really cold, that sweat is going to freeze. Even if it doesn't freeze, the evaporating sweat will give you hypothermia.

I prefer lightweight baselayers to heavy ones. The idea of the baselayer is to wick the moisture away from your body rather than to insulate. Your insulation should be a number of layers on top of the baselayer. You want to be able to fine-tune your body temperature while you're moving to avoid sweating. When you stop to rest or make camp, you need to put lots more insulation on, and that's when the heavier insulating layer(s) (like the puffy jacket and pants) come into play.

Gaiters actually work very well to keep snow from going inside your boots. I wouldn't go out in snow without them!