The i5 is the preferred CPU for games over the i7 because games do not take advantage of Hyper Threading (HT), so why pay the extra $$ if it's something that is not going to be used? Also, benchmarks have shown that HT reduces game performance by 1% or 2%.
Generally speaking, the higher the resolution, the more performance is dependent on the GPU rather than the CPU. At 1920x1080 resolution or higher most CPUs would provide about the same performance level, unless the game is CPU dependent as well; such as Skyrim.
However, you should look at benchmarks to make your own decision; like the following...
http://www.xbitlabs.com/articles/cpu/display/amd-fx-8120-6100-4100_4.html#sect0