Unlike some select (and usually expensive) notebook makers that design high end notebooks for upgradeability (Origin, Sager, certain Asus and MSI models, etc.) the Macbooks (depending on model mind you - certain models are fixed at purchase and can't be upgraded) can only have RAM and HD upgraded/replaced.
If you're wanting the Mac experience but have a decent gaming experience, then you'll need to move to the Macbook Pro as already mentioned with the 650m GPU. I wouldn't get the Retina display version but rather the more upgradeable (and lower resolution) non-retina version.
I have the non-retina version and most games I can play at between 720p and 1080p without too much trouble (depending on game and resolution chosen will determine the rest of the settings, but anywhere from low/medium to medium/high and maintain a decent framerate). I have an external Thunderbolt display (2560x1440, which is lower res then the retina @ 2880x1800) which is really too high of a resolution for the 650m to play games on with a decent framerate with even low settings. Sacrificing a little resolution for me (or native resolution on the non-retina MBP) you can get really good performance.
I play a variety of games on mine including BlackOps 2, Guildwars 2, World of Warcraft, Path of Exile, Deus Ex HR, Eve Online, Batman Arkham City, Bioshock, Rage, etc. As long as I'm willing to play with my settings a bit on my screen I get get good performance. Many games I run at 1600x900 which is higher res then the 1440x900 on base MBP 15" non-retina and a bit lower then the 1680x1050 high res display on the mbp non-retina with the optional display upgrade (which I do have, when running in laptop mode, I usually play just about everything at 1680x1050 except a few games like Metro 2033)