Well, you should be able to run games like Dragon Age 2, Starcraft 2 or Diablo 3 on low-to-mid settings just fine. (I'm talking about a 1080p-Screen here - on a 1440p-Screen they may only run in low settings). Games like Metro 2033 however shouldn't run smooth.
As for App Store games - there are some "real" games on the App Store like Bioshock or Duke Nukem Forever. Both should run good in mid-to-high settings. So these are no problem, at least this year. More demanding games next year or even 2014 may be a problem, though.
Oh, and here you can find graphical benchmarks about the HD6630M, which should be adequate for your Mini (for games you play on OSX, take 10fps off - the graphic drivers on OSX aren't too great for games). Just scroll down to the game benchmarks.
http://www.notebookcheck.com/AMD-Radeon-HD-6630M.43839.0.html
As for App Store games - there are some "real" games on the App Store like Bioshock or Duke Nukem Forever. Both should run good in mid-to-high settings. So these are no problem, at least this year. More demanding games next year or even 2014 may be a problem, though.
Oh, and here you can find graphical benchmarks about the HD6630M, which should be adequate for your Mini (for games you play on OSX, take 10fps off - the graphic drivers on OSX aren't too great for games). Just scroll down to the game benchmarks.
http://www.notebookcheck.com/AMD-Radeon-HD-6630M.43839.0.html