Well, I'm not 100% sure, but I don't think that there's anything other than directly plugging the card into your montherboard that would work very well. One of the constant bottlenecks in modern computing is memmory. The video card needs to get data from the computer as quickly as possible, and USB would probably be WAY too slow. The only ways of hooking the card into your computer I know about are AGP and PCI express.
Hmmm... and part of that NVidia link mentions PCI express.
Hmmm...