Combine sugar and cornstarch in a saucepan. Stir in raspberries and jelly. Bring to a boil over moderate heat. Reduce heat to low and continue to cook and stir 5 minutes until smooth and thickened. Press sauce through a sieve; chill thoroughly. To make Melba, place a small scoop of ice cream in the bottom of a sherbet glass. Cover with a peach half. Top with raspberry sauce and sprinkle with macaroon crumbs. Garnish rim with a fluting of whipped cream. Serves 6.