Okay, I'll say it.
Why spend hundreds of dollars upgrading your Mac to still be underpowered for these games? Why spend thousands of dollars getting a new Mac capable of running these games properly?
Spend about $300 for an Xbox, plop your ass in your recliner, put your feet up, and game on your 4 foot wide television using your wireless controller. Those games are guaranteed to have satisfactory framerates, and you'll save bundles.
Just sayin' . . .