I haven't heard of any external GPUs that are available. Not to mention the limitations of connection interface that would be imposed on such cards (USB is not the way to go here)
essentially this is what you're looking for:http://www.amd.com/us/products/technologies/ati-xgp/Pages/ati-xgp.aspx
that came about in 2008 and has died since then, afaik. there are some 3rd party solutions out there, but all of them involve your laptop having a special port to begin with.
so far, you're out of luck, if you want a good GPU in a laptop it needs to be on the motherboard.