It's not just about looks. Simple version is this: Your screen resolution directly determines how many pixels your GPU has to render when playing a game. Low resolutions means the GPU has less work to do. This is why on game reviews, if they want to test the CPU load, they set the game resolution as low as possible to shift the work towards the CPU and away from the GPU.
If you buy a very powerful GPU but play at low resolutions, most of the time the GPU will have nothing to do. It actually needs a higher resolution so that it can stretch its legs a bit, so to speak. Thus, any money you spend on a GPU upgrade in a case like this will be wasted. You won't notice any difference in performance.