i would not recommend it with a imac or a mini , they are just no gaming machines , its not so much about the processing power , its more a thing of the graphic cards are not really suitable for gaming , and as you physically cant upgrade them to top graphic cards
it has to be a mac it has to be a mac pro where you get the optimum processing power and you can upgrade graphic cards , but then you will be cheaper of with a pc , as for most demanding games you need windows anyway to play them nowadays , so there is no point of buying a mini or a imac if you want to do serious gaming you simply need a pc
if i would be you i would go for a i7 processor overclock it to about 4ghz use a nvidia quadro plex graphics card and a bit of ram rouhgly absolute minimum for games i would suggest about 12gb ddr3 under that i would not even try to play any demanding games and you need a 10000 rpm harddrive ,
you could go with less but then it would be a compromise, there is a reason why serious gamers spend more then others pay for a car on their gaming machines