Preheat the oven to 325 degrees Fahrenheit.
Using a food processor, finely grind the graham crackers. Pour the crumbs into a bowl.
Measure out the sugar, and pour it into the bowl with the crumbs.
Melt the butter in the microwave or in a pan on the stove top. Pour the butter over the crumbs and sugar, and mix thoroughly.
Spoon the mixture into a 9 inch glass pie plate, and press down firmly and evenly across the bottom and up the sides of the dish.
Bake the crust for 8 minutes. Then remove from the oven, and set on top of a wire rack to cool.
Raise the temperature of the oven to 350 degrees Fahrenheit.
In another bowl, combine the condensed milk, key lime juice, sour cream, and lime zest thoroughly.
Pour the mixture into the cooled pie crust, and bake for 7 minutes, being careful not to brown the top.
Remove the pie from the oven, and let it cool completely on a wire rack.
When the pie is cool, place it in the refrigerator until chilled.
Serve as is or with a dollop of whipped cream on top.