Your "bottleneck" will be dependent on the software you run and your display.
For gaming:
Your CPU handles the game engine and will be able to make a certain amount of frames per second.
Your GPU handles the pixel shading and post effects based on resolution and detail settings and can make a certain amount of frames per second.
Your monitor has a resolution and a certain refresh rate. Ideally you want your FPS to match your refresh rate. Whichever of the CPU of GPU cannot handle the workload to make said FPS in that game/application then it is the "bottleneck".