Bottlenecks are not simple, and it will depend on the task as to which is the limiting factor. In a game that's CPU heavy, your GPU utilization will be less than 100% because your CPU is what's holding it back. In games that are graphically demanding but not very CPU heavy, your CPU utilization will be less than 100%, and your GPU will be what holds you back - though in this case, you can lower your graphical settings to get higher framerates.
It's kindof a moot point though, because most likely, neither is upgradable.