Actually, USB2 does have enough bandwidth. I'm using a Tritton Technologies SEE2 USB to VGA adapter for driving a secondary monitor at resolutions of up to 1280x1024. The driver also has a mode to mirror whats on the primary display.
I bought mine here.
I have had some driver stability issues, but it may be because I've primarily used the SEE2 when debugging programs from Microsoft's Visual C++ development system - e.g. it may be the debuggers fault.