All you guys have to do is read the documentation.
These are not 'video cards on USB'. Think of them more like a USB-to-VGA adapter.
What they are is a way to compress video from your video framebuffer over USB and have it display on a VGA capable display. The USB device is little more then a FPGA chip designed to decompress the video stream.
There is no graphics processing or anything like that going on in those USB devices other then the decompression of the stream.
If there is any video acceleration going on at all then it's happening with your onboard GPU. I believe they have support for accelerating the display using Intel IGP, but I don't know for certain since I don't own one. As far as AMD or Nvidia support, I have not the fuzziest clue.
That is why these things are so cheap and are so effective. There have been 'USB Video Cards' in the past and some chips did work in Linux, but the approach that the displaylink uses is much better performance-wise and can drive much larger displays.
The video included in the article is a testament to that.