In short, you want a CPU that is equal to or greater than what the GPU pumps out. If you have it inverted you can experience black screens or crashes if it too severe.
STABLE = Powerful CPU >= GPU output
UNSTABLE = Weak CPU < GPU output
For the best performance fit the GPU performance inside of what the CPU can handle.
PERFORMANCE = Powerful CPU = GPU output
OK = CPU > GPU output
There is a handy tool to check this:
Sometimes you will find that you are getting CPU bound when a game isn’t optimized for multi-threading. You may also find that if you’re using multiple graphics cards is that you will not have an even distribution of GPU across multiple cards. This is dependent on the game itself and its architecture.
Depending on which games that you are interested in playing it is a good idea to check what the distribution between CPU and GPU usage that those games use.