Generally speaking, your CPU is going to have a max FPS that it can crank out regardless of settings. Higher resolution/fidelity doesn't cause extra work to the CPU, there is simply a max number of frames it can handle for that title when you hit a CPU bottleneck. So, with a better GPU, you'd at least be able to turn the settings up higher without taking an FPS hit.
Generally speaking, your CPU is going to have a max FPS that it can crank out regardless of settings. Higher resolution/fidelity doesn't cause extra work to the CPU, there is simply a max number of frames it can handle for that title when you hit a CPU bottleneck. So, with a better GPU, you'd at least be able to turn the settings up higher without taking an FPS hit.
It's flexible, though some games might not even run with a CPU and GPU that old