There are actually a few products that have an external GPU in a box that communicates with the laptop via the express card slot. The best GPU you could get for it is a 4670, though, due to bandwidth issues.. and you'd need an external display too. They're also very expensive.
Tom's even has something on one of these products, the ViDock. (article is from last year, current-gen midrange cards are available now)http://www.tomshardware.com/reviews/vidock-expresscard-graphics,1933.html