You can't do real gaming in parallels(it will run some older games, but newer stuff doesn't work). If you want to game on a mac, install bootcamp+windows or buy "universal binary" mac games.
Most iMacs have midrange graphics cards(ati radeon hd 2400xt or hd 2600 pro) installed, so they should handle normal gaming uses, so they're not completely "useless for gaming." At least they have a graphics card, unlike the piece of crap dells "everyone" buys.