Hey Zimmmmmmy,
Just to clarify, the tank with the radiator cap is not the overflow tank. It's the reservoir tank. There should be a hose coming off the reservoir tank that leads to the overflow tank... on my car its a squarish plastic tank under the front passenger fender. If you do not have an overflow tank, I highly recommend one. It can be any thing, (a milk jug, a large coke bottle... whatever). Just make sure that the tube that comes from the radiator cap goes all the way to the bottom of the overflow tank.
The overflow tank will catch the coolant that blows out the radiator cap and store it until the car cools and sucks it back in again. It won't really solve a chronic overheating problem, but it will stop your car from slowly blowing out all its coolant until you have none left.