Cook potatoes in large saucepan of boiling water 15 to 20 minute or until potatoes are tender; drain. Return potatoes to pan. Mash with hand masher until smooth, gradually adding milk and mixing well after each addition. Stir in cheese. Spoon into 2-L casserole dish.
Mix cracker crumbs, garlic powder and margarine; sprinkle over potato mixture.
Bake 20 to 25 minute or until potato mixture is heated through and topping is lightly browned.