Line the bottom of a buttered 13"x9" baking pan with a single layer of crackers; set aside. Melt margarine in a heavy saucepan; add sugars, graham cracker crumbs and milk. Heat over medium-high heat until sugars dissolve, stirring often; spread over crackers in pan. Arrange another single layer of crackers on top; set aside. Combine remaining ingredients in a saucepan; heat over low heat until melted, stirring until smooth and creamy. Spread over crackers; set aside until firm. Cut into bars to serve.