Peel and quarter potatoes. In a large saucepan, cook potatoes in enough water to cover for 20 to 25 minutes or until tender. Drain off liquid. In a large mixing bowl, combine the potatoes, cream cheese, sour cream, butter/margarine, garlic powder, and pepper. Beat the mixture with an electric mixer till fluffy. Turn mixture into a buttered 2-quart casserole or baking dish. Top with cheddar cheese. Cover and bake in 350° F oven for 45 minutes. Uncover and bake for 15 minutes more. Serves 8.