think of a computer's cpu as its brain.
think of cpu speed or clock as how fast the brain thinks. The brain thinks the whole time, but when you're playing a game, it has to think alot harder (depending on which game though).
The faster the brain thinks, the faster it can access your 512mb of vram. Hence, the faster the performance.
Slow brain = slower accessing of vram = slower performance.
Not taking into account any other variables which may affect video performance, that is how the cpu speed limits the capability or potential of a graphics card.