Last I checked, almost all PC games were not noticeably faster with more than 1 or 2 cpu cores (the engines weren't very multi-threaded, apparently).
Are most games and game engines coming out now able to take advantage of more than 2 cores?
(If not, can you suggest a few that do?)
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.
If you look at it logically: Speedup in computers is done through parallelizing only (at least significantly). If you want to publish a game that takes advantage of current computers (and that's what most games want to do) you have to make your game ready for multithreading (see also this blog post).
Obviously parts like physics engine or KI can easily run each in a single process (if written like that!) which makes it easy to at least use some of the additional cores.
