In a small heavy saucepan combine sugar and water and bring to a boil, stirring until sugar is dissolved. Remove pan from heat. In a blender or food processor puree raspberries with Champagne, creme de cassis, and sugar syrup until smooth. Force puree through a fine sieve set over a bowl, pressing hard on solids, and stir in lemon juice. Chill mixture until cold and freeze in an ice-cream maker. Makes about 1 quart.