Can I add a dedicated graphics card to a mac book pro 13" 2011?

If you want to do gaming then stay away from Mac OS X since almost no popular games are available for it unless you dual boot Windows. Apple also doesn't use AMD processors either; they use Intel processors. Apple does use AMD graphics processors. You also can't install a better graphics card in the machine regardless of which MacBook Pro you get; you'll void your warranty and possibly make you system unusable since Mac OS X checks to see if all the correct hardware is installed. The Intel Sandy Bridge processors (or just the next iteration of the i-series of processors) don't have much of an effect on gaming since it's just a processor. Bottom line though, Mac is not a great machine to get for gaming; you can build a legitimate gaming computer for about the same price you would spend on the Mac and get 2GB of dedicated video memory and faster processor and more RAM if you're a smart shopper and have that machine be 3x better than the Mac in every regard. Stick to Windows for gaming.