Bottlenecks aren't simple. There are cases where a monster 10 core i7 6950x will limit framerate even with something like a GTX 750. Conversely, you will also find cases where a GTX 750 is a the limiting factor even with a decade old Core2 machine. Faster is always better, and GPU and CPU requirements and load in games are largely not related to each other. Generally speaking, it's a good idea to build a system that's "balanced", but what the correct balance is will depend on your uses and expectations.