That depends on the games, output resolution, detail settings and your personal definition of how good is good enough.
Games with low CPU requirements like Diablo 3 would play fine even on a Pentium or even a Celeron while games with heavy CPU requirements like multi-player BF4 can show some performance scaling all the way to i7-4960X.
sweet spot is the i5 K. if you run too close to no bottleneck you could just end up with them both bottlenecking at different times. It would also mean that if you update you gpu you'll get zero benefit and you'll have to update all at once.