Preheat oven to 375F. Bake pastry shell 10 minutes; remove from oven. Reduce oven temperature to 325F. In saucepan, over low heat, melt chips with margarine. In large mixer bowl, beat chocolate mixture with remaining ingredients except nuts until smooth. Add nuts. Pour into prepared pastry shell. Bake 35 to 45 minutes or until center is set. Cool. Serve warm or at room temperature with ice-cream if desired.