Yeah--you can't get there from here. The USFF has no expansion slots whatever, so what's on the motherboard is what you got. Due to its compact size, 'Optiplex 760 USFF' is not designed with an extra card slot for a discreet GPU. Therefore, it does not allow video card upgrade by any means. An external GPU, if used will still be connected to the integrated card and will not produce improved performance.
So there is nothing you can do with any Dell model of USFF in terms of video card upgrade.