The important thing to consider with USB is that it just can't move bits
fast enough to deliver a high-quality video signal. USB capture devices
are ok for little video mail clips and playing around, but if you want to
get any sort of good results at all, you have to have either an analog
capture card or a firewire port inside your computer, plugged into a slot.
USB will limit you to tiny frames, slow frame rates, or drastic compression
that will distort the video signal badly.