Yes.

The long answer: If you want a computer just for gaming, you're obviously better off buying a gaming PC instead, since it's likely a lot cheaper and more upgradeable. If you really want a Mac for everything a Mac offers, plus want to do some gaming every now and the, the iMac will be just fine for you. The current crop of graphics cards in the iMac 2.8Hz are up to the task, but are not what you'd find in a high-end pure-breed gaming PC. There's a few other things to keep in mind too, one being that you'll have to reboot into Bootcamp each time you want to play and back to OS X when you're done (some few games also run in Parallels Desktop, but not nearly as fast). Another thing to remember is that Mac keyboards are slightly different from PC keyboards, so some keys will be different (alt/cmd) and others will simply not be there (Pause, scroll lock, etc.). You can either remap these under windows with some extra software or just attach a PC keyboard for your gaming needs (any USB keyboard will work).

Perhaps it's also an option to not spend as much on the Mac (depending on what you want to do with it) and get a cheaper gaming PC in addition to it, hook them up to the same screen and share keyboard and mouse via Synergy or similar methods.