Almost all new engines or recent engines have been programmed with the multi-core in mind.
I am speaking about Unreal Engine 3, Source Engine, CryTek Engine, ID Tech Engine and more.
So, if you would asked this question 2 years ago, I would answered you to go with dual-core CPU and get the highest clock frequency that your budget would allow you.
But nowadays 4-core is the way. Of course puzzle, indie or casual games will not benefit too much from multi-core. But FPS, RPG and even RTS games will do.
Choose an Intel i7 is not necessary. Intel Core i5 and AMD Phenom II X4 are great for games, bundled with a good GPU (of course).
The rule of highest as-your-budget-allow CPU clock is still valid in the 4-core CPU selection. But consider that Intel compared to AMD at the same clock frequency will win.