Personally - None. Far too dangerous a task if possible at all with your unit. You are stuck with what came with the laptop. There are some USB graphics cards but they are no good for gaming.

Theoretically it is posible but I wouldn't do it and I work in IT and with hardware. You would void your warranty without any doubts. Plus I would have no idea where you buy one like that for it. It would only work on units like Dells who had the upgrade option available for the initial purchase.

Here is a guide:
http://www.pcworld.idg.com.au/slideshow/255471/upgrading_your_laptop_graphics_card/?image=1

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