To do any "real" gaming, you generally want a discrete graphics card - a graphics card that is separate from the CPU, with its own GPU chip and memory. The iMac you're using has "integrated" graphics, meaning there is a portion of the CPU dedicated to graphics, which uses a portion of the system RAM for graphics memory. This is common in PC's (by which I mean computers) with space limitations, such as laptops or "all-in-one" PC's like the iMac.
So basically, the question is, which AAA titles are the least demanding graphically. (Not nessessarily least demanding overall, since you have a very capable CPU, and I assume lots of RAM) CounterStrike and LeagueOfLegends come to mind, and older games.