It's crazy how long we've had quad-core processors, but a cheap duo core processor can perform nearly the same for most games.There are some already, Civ V is n-threaded.
The rule of thumb right now is basically that the more threads a game uses, the less reliant it is on CPU performance in terms of affecting average frame rate. (assuming a modern quad core is in use). Even still, games that are well known to be multi-threaded, like Crysis 3, are still all about that IPC. The Intel Pentium Anniversary at 4.8GHz keeps up with the 2500K and 8350.