I have a video card in my pocket
EVGA is releasing two new products, called the UV12 and the UV16, which will make setting up multiple displays in a hurry much easier.
EVGA says that these new UV Plus+ series USB 2.0 devices have a "GPU-less design." Instead, the UV12 and UV16 make use of a virtual driver linked to some DDR memory. The products are capable of displaying output at a high resolution (1440x900 for the UV12, and 1680x1200 for the UV16) and have enough processing power to run MS Vista's Aero interface without any problems.
The UV12 and UV16 seemed to be aimed primarily towards those who sometimes need to setup multiple displays quickly -- say, for a business presentation using PowerPoint or something similar. Multiple units can be stacked because they are magnetized, and they also come with software to easily rotate and extend the displays.
The product page of this interesting new device is over here. The more powerful of the two products, the UV16, will be selling very shortly for $90.00 USD.
I imagine when USB 3.0 comes out, you'll see more powerful external Video Cards, actual video cards, instead of software based ones.
At least, I would think it should be possible...