Support for usb vga cards is now in the 2.6 kernel, however there are some devices out thier that suposidly "require" usb 20. and wont work unless it connects to a usb 2.0 port, this seems a bit absurd to me but i found this infomation floating around on then net (isnt it a requirement for all USB certified devices to be backwards compatible)
frame rate wise it should be posible to do about 18 frames a second @ 640*480, 16 bit color