Nope, a CPU's job in gaming to just to figure out all the background things like enemy AI, physics and crunch code. What you see on the screen is entirely up to the GPU.
In a CPU bottlenecked situation, you will see a performance increase by upgrading it. But as long as you have any modern dual core processor (basically a Sandy i3 or better) then in most situations your fine for gaming. The GPU is what really drives up FPS and detail settings.