In a medium saucepan, combine the sugar and 1 1/2 cups of water over moderately high heat; stir to dissolve the sugar. Just before the mixture comes to a boil, remove from the heat. Stir in 3 1/4 cups of cold water, then pour into a pitcher. Stir in the lemon and lime juices and refrigerate. Serve in tall glasses over ice, garnished with lemon and lime slices.