In a medium saucepan, combine peaches, sugar, and 2 cups water. Bring to a boil over medium-high heat; reduce heat, and simmer until peaches are very tender, about 20 minutes. Strain mixture into a bowl. Cover, and refrigerate for at least 4 hours before using.

Pour about 1 1/2 tablespoons peach syrup into each Champagne flute. Top with chilled Prosecco and raspberries. Serve immediately.