Simple Syrup: On the stovetop in a small saucepan over medium high heat mix together sugar, 1 cup water and 1/2 cup lime juice. Heat, stirring until sugar has fully dissolved, then remove from heat.
Limeade: Add simple syrup to a 32 ounce or 1 gallon pitcher with 4 cups water, 2 reserved cups lime juice and sliced lime. Cover and chill for at least 4 hours.
Just before serving, add ice and serve with additional lime slices and mint for garnishing, if desired.