I don't get why people do that, they buy a laptop they know they can't game on, they then they ask whats the best GPU upgrade they can get. When by the time you have spent the money on a new GPU. you could have just gotten a better laptop with the GPU you want to upgrade to already on it for the amount you spent
I was about to link you an external usb3.0/esata gpu solution but unfortunately according to this: http://pixelsmashers.com/?p=1895 this isn't a reliable option.
The only thing you can do is to buy a cheap gaming desktop and keep the laptop for its portable use.