Well, the CPU is the "bottleneck" in the sense that if you had a faster CPU you'd get more FPS, but generally speaking, as long as you are getting over 40 FPS, you are doing good. Your CPU is able to handle the game well enough that you are at a perfectly reasonable FPS, and if you increased your resolution, you'll CPU would still be fine, and would allow your GPU to work harder.
Unless you don't want a higher resolution, your next upgrade would best be a monitor of higher resolution, and likely size. If you are happy, a faster CPU will allow for more FPS, but you probably wouldn't notice much if any difference.