No, macs are not good for gaming. They are adequate. But they won't play latest games at max settings/1080p with 60fps if that is what you are after. It is very unlikely that the Mac Pro will have SLI. And it won't be 'good' for gaming either, as it will use workstation GPUs. The iMac's top GPU is roughly comparable to the GeForce 660Ti, which a decent card, but definitively not on the top. If you are looking to maximise your gaming performance, a Mac is a wrong choice.
That said, even the 650m can handle games with 'intense graphics'. You just need to lower the resolution/tweak the settings a bit. I am very happy with my 15" retina MBP in regards to the gaming performance, but there are people who would find it completely unacceptable.