USB and PC-CARD Video cards and more discussed at http://reviews.cnet.com/5208-7813-0.html?forumID=96&threadID=131549&messageID=1484032
Bottomline? It doesn't help if you are trying to fix a laptop that doesn't have a great 3D chipset. But do read the discussion so you will know what is out there.
There are also USB to PCI and PC-Card to PCI adapters, but these are too slow for gaming and card support is non-existent in many cases.
I'd like to plug a PCI card on my laptop. Are there some adapters allowing to plug a PCI card in USB or PCMCIA (others ideas ?) ?