The idea of balancing your system isn't so much about bad and good, but more about being wasteful.
To know if your CPU is good enough, you'll want to just test out games set at their lowest settings. If they are still unplayable now, it probably wouldn't matter what GPU you put in it, the CPU just can't function well enough for gaming.
Picking the right GPU is generally best determined by what resolution you play at, but also the games your CPU can handle. If the CPU can't handle modern games, then you need to judge the GPU requirements on the games the CPU can handle at the resolution you are playing at.