In a large bowl combine graham cracker crumbs and confectioners' sugar. Add melted margarine and peanut butter, and mix well with a wooden spoon. Add walnuts. Spread evenly in an ungreased 10x13-inch jelly roll pan.
Make Topping: In a small heavy saucepan melt margarine. Add chocolate chips and stir over low heat until completely melted and smooth. Spread over the peanut butter mixture. Chill pan in the refrigerator. Cut into 1-inch squares.
