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15-inch MacBooks Pro have got two graphic cards. It has got one that it's used by default, an integrated card (Intel HD Graphics 4000) that provides a basic graphic performance for normal tasks, and then, a dedicated graphic card (in the case of the new MacBook Pro, a NVIDIA GeForce GT 650M) for heavy tasks, like Final Cut Pro X, Photoshop, games...
Your Mac changes the graphic card automatically when your Mac needs more performance, so you don't have to worry about it. If you want to change the used graphic card manually, you can download gfxCardStatus > http://gfx.io
You have 512 MB of dedicated memory because you have the 15-inch non-Retina MacBook Pro. The memory isn't the most important feature of a graphic, but it may provide more performance