Usb graphics adapter as stand alone display adapter


Does anyone know if you can use a usb graphics adapter as the only graphics adapter in a computer? 

What I want to do is add another NIC to my vm server but to do so I need to take the graphics card out. So I figure I can get a usb adapter to run the display for when I need to use it. 

But I can't find any info if these things work like that, or if they only work in conjunction with a real graphics card to add extra displays. 

Also does anyone know if they work in Linux?