i wouldn't rely on usb for video. The bandwith is simply not sufficient. Tricks of compression might passably make it work... but that prob means other side effects such as maxing out your cpu.
Here's informative link about device bandwidths:
it used to be simpler where you can get a feel of comparative speed of different devices (scsi, usb, firewire, ethernet, vhs, dsl, pci, ram etc). But now the page is quite rich and categorized and detailed but harder get a over view.
Here's a simpler table i compiled in the past:
• Digital Communication Bandwidth