Fallout 3 is a great benchmark to demonstrate how average FPS alone doesn't tell the whole story. At 1280x1024, look at the difference in average frame rates on the Radeon HD 4650 cards. They don't change much with the CPU overclock. However the minimum frame rate gets a massive boost from the faster CPU. Minimum frame rates are more important than average frame rates here because they represent the worst-case scenario (how slowly the game runs when things get busy on the screen). Choppy performance during intense sequences most certainly sours the gaming experience, even if averages trend higher.
At high settings with 4x AA and 8x AF, the CPU overclock accomplishes a lot less. This is because the graphics cards have become the bottleneck when these image quality enhancements are turned on.