iMac and hardcore gaming are not words one usually associates with one another.
Seriously, I love my iMac and do play games on it (Boot Camp) but the GPU is relatively average (generally Apple use mobile variants in the iMac due to heat limitations) and would struggle with maxed out settings on the most modern games. Don't get me wrong, your iMac will play most of these games and older games you can go pretty high in settings (although I avoid anti-aliasing settings altogether).
If gaming is your priority then (and I thought I'd never hear myself say this) avoid an iMac, for around half the price you could get yourself a PC which would out perform the iMac and, more importantly for a gamer, allow you to upgrade the GPU at a later date.