Wash, peel and cook potatoes until soft. Turn in to an electric mixer and add sour cream, cream cheese and margarine (or butter). Mix until smooth. Spoon into a greased 2 qt. casserole dish. Bake at 375 degrees for 45 minutes, or until bubbly. The depth of the casserole dish may cause a variance in the baking time. These potatoes can be made days ahead of time and refrigerated. Perfect for the holidays!