In certain games like crysis 3 I get a huge boost in performance from overclocking my GPU and others not so much. Sometimes a overclock can make the difference between playable and non playable FPS.
For the CPU, it is suggested/highly recommended to have an aftermarket cooler on it. For the GPU, It doesn't matter as much, most gpus you buy has an aftermarket cooler that's decent enough for at least a small overclock.
In certain games like crysis 3 I get a huge boost in performance from overclocking my GPU and others not so much. Sometimes a overclock can make the difference between playable and non playable FPS.