There is no such thing as a "perfect clock speed". All games have different requirements and will weigh on the CPU in varying amounts depending on the game and context. For instance in a RTS game like Supreme Commander where there are a lot of AI bots and tons of things going on in the game, your CPU speed will be paramount and the faster CPU you have the better the game will run. However in games like Metro 2033 that are so graphically intense and use piles and piles of advanced GPU techniques, a faster GPU will be of more value and give you better performance.
 
There is no static formula for a perfect balance between CPU and GPU. Your best option is the test your setup in the games you play and adjust your clock speeds accordingly.