Tech Q: Graphics Card & Processor

[Vaya]

What is the CPU's job when it comes to video games, if I have a high end graphics card, and a Pentium 4 processor for example, how would that affect my gaming experience vs. Low end graphics card and high end processor?

Spoiler!

Story behind the question: My ATI Radeon HD 5400 graphics card might not be the best but it's good enough to run most of nowdays games, while my Pentium 4 3.2 GHz CPU is always under the minimum requirements, but some games still run smoothly because of the graphics card I guess, like Assassins Creed Revelations, which can run on Max graphics setting with no lag at all, some other games like Need for speed hot pursuit's fps drop to ~5 or something, so I'm really curious to know which part affects the performance more, and how every part works when it comes to gaming.

[KillAndChill]

Depends on the game. SC2, for example, requires a good CPU but will work with a 6 year old card.

[CrimsonNuker]

Low end CPU + High end GPU = GPU gets bottlenecked
High end CPU + Low end GPU = CPU gets bottlenecked

IIRC. but that's all I can explain

[Rawr]

A game such as WoW or most MMOs would benefit from a high-end CPU, not so much from a GPU.
A game such as Metro 2033 or BF3 wouldn't benefit a whole lot from a ridiculous CPU as it would a high-end GPU.
Really what works all around is a decent quad core and any mid to high-end GPU.