CPU for gaming. Core Dual / Quad core?

If I am playing a high quality game (Let's say the game doesn't support multi-core, actually I don't know currently what game supports multi-core ) with Core Dual E6850 with suitable graphic card like HD3870, and play the same game same game settings with Quad core Q6600, same with HD3870, is there any difference?

Will the difference in clock speed make a big difference?

Or will the Quad core can allocate a whole core to handle the game and allow the rest of 3 cores to handle the background jobs while the core dual has to use part of the game-handling-core to work with the background job?

Thank you