The GFX function is to render the scene.
The CPU, Memory, Drives function is to direct the scene.
Both are critical to a satisfying and tasty gaming experience, IMO.
OC'ing one particular part of the system often will reveal bottlenecks elsewhere in the system.
Also, hardware can only run as well as the software which it is running, some software (games) require more CPU 'horsepower' just because it is bloated and ill concieved to start with.