In my experience (sorry to refute), the GPU is more important than the CPU... this is apparent with many games. Yes, as @iQQator said, the CPU can help bring out more details and allow more things to occur at once, including more things physically happening in the simulator, but the GPU is responsible for a lot of the visual effects that go on, such as generally make the textures and simulator as a whole more appealing to look at and actually drawing and sharpening all of the textures, lines, reflections, shadows, etc.
See this link: developer x-plane cpu vs gnu
If my terminology is correct... basically, the CPU is like a drawing guide... it says "Hey, everything goes here", or "this goes here", and "let's put some more of these here", and the GPU actually draws most of the stuff, saying like "Okay, lets make these textures nice and crisp", or "lets make some shadows for these", or "lets add some reflections".
^^^ that was a pretty dummed down (and probably slightly incorrect idea) of what goes on. Just read the link. I tried lol.