I've changed my rating from 4 to 5 stars because the more I make this the more I realize just what a winner Harriet has given you and you, in turn, have given us. I use regular sour cream, 2% milk, and 4 oz of cheddar and 4 oz of jack cheese. Must say though I enjoy this dish better the next day at room temperature.
A friend brought this to a potluck and then gave out the recipe, which is identical to the one posted here. It received more raves than any other dish at the potluck meal, and there were many good ones.