Grease a 10x15 inch pan. In a large bowl, combine 1 1/2 cups melted margarine, peanut butter, confectioners' sugar and graham cracker crumbs. Spread in prepared pan.
Combine 1/2 cup margarine and chocolate chips in a medium saucepan over medium-low heat. Stir occasionally until melted and smooth. Spread over peanut butter mixture. Let cool completely before cutting into bars