Preheat the oven to 300 degrees.
Pour the pumpkin filling over the prepared crust. Tap the pan down on the counter a few times to settle the cream cheese and remove any air bubbles.
Place the foil wrapped pan in a large roasting pan or baking dish. Pour enough hot water into the roasting pan to come about 1 inch up the sides of the springform pan.
Bake for 1 hour.
Reduce the oven temperature to 275 and continue baking 1/2 hour more until the center is set.
Remove the cheesecake from the oven and spread the sour cream topping evenly over the top.
Place the cheesecake back in the oven, still in the water bath, and continue cooking 7 minutes more until the sour cream topping is set.
Remove from the oven and roasting pan and cool the cake completely.
Refrigerate 6 hours or overnight before serving.
You can also freeze the cheese cake in the springform pan, well wrapped with plastic. Thaw completely in the refrigerator before serving.
To serve, remove cake from the pan, and garnish with chopped nuts, caramel sauce or whipped cream if desired.