As an example, I play Guild Wars 2 which loads 1 core to 75% or so and 3 more cores to 50% or so. This maxes my GTX 970 before it maxes my CPU. This can vary with detail settings though.
As iLiftFood mentions, older games that only use 1-2 cores will cap on the CPU befpore the GPU.
Current/modern games normally use 4+ cores now and will be fine.
I've seen this prove true on an FX 8350 @ 4.5GHz and a new FX 9590 @ 5.0GHz. A single 970 or 980 will be fine for these systems.