I have a pair of Lacrosse Burly 1200 gram rubber boots. With a light sock they keep my feet toasty all day till the temps dip below 40 during the day.

I'll add a wool/polypro sock over a light poly liner sock and I'm warm down to about 30 degrees. Just don't try hiking like that or you'll wind up with "squishy" feet!

Make sure that your boots aren't too tight either. Restricting movement discourages blood flow which is what what keeps your feet warm the best.