In 1 1/2-quart saucepan, heat water and sugar over medium heat about 2 minutes or until sugar is dissolved. Cool completely, about 30 minutes.
In 8-cup blender, place sugar water, lime juice, rum and mint leaves (if smaller blender, do in batches). Cover; blend on high speed about 20 seconds or until mint is finely chopped.
Pour mixture into 13x9-inch (3-quart) glass baking dish. Freeze 4 to 6 hours, using fork to break apart ice crystals every 2 hours.
To serve, spoon 1/2 cup mixture into each glass; pour 1/4 cup ginger ale over each. Stir. If desired, garnish with mint sprigs.