There's no straightforward answer to this. It depends on how well the chip's single threaded performance is.
For example, a 3.5 GHz Intel i7 quad core performs better in most games than a 4.0 GHz 8-core AMD, because it has better single-thread performance. It really depends between which two you are picking between. Your graphics card will be a bigger impact on your gaming performance than your CPU will, but a bad CPU will bottleneck you.