I wouldnt recommend it, too much a hassle. If you want to take a look at it have a read at this. Bear in mind you'll need a ExpressCard slot, a spare stable PSU, a PCI-e adapter and a graphics card.
For that it's just better to get a new $600-$800 dollar build.http://www.techradar.com/news/computing-components/graphics-cards/how-to-make-an-external-laptop-graphics-adaptor-915616
EDIT: Also external video cards wont necessarily be supported by any game.