I like Duke's suggestions regarding proper layering for sweat control on your feet. If you don't control that then it doesn't matter how many grams of thinsulate you have in there....

Personally, I am a Cabelas man...<img src=icon_smile_wink.gif border=0 align=middle>. I usually head over to their bargain cave and check what is on sale...If you are looking for extreme cold weather boots then don't get anything under 1000 grams of Thinsulate.....waterproof ofcourse....preferably Goretex or something similar.