It seems games have been using multiple cores for some 5-6 years now. They all will use multiple cores these days unless the dev does think they need to put in the effoort. At least two cores, but as far as I've seen only the latest will really fully use 4 cores.
I'd recommend getting a fast quad core cpu. The i5 4690k is a beautiful CPU for gaming, not too spendy but the cores get an incredible benchmark score.
I'd stay away from 8 core CPUs and avoid 6 core CPUs, they really don't give much benefit at all. Any good quad core can handle basic multi-tasking and gaming at the same time.