Original display is configured VGA. Additional display is VGA and DVI. Back of the 790 is VGA and DFP. Cable connectors look to big to fit alongside one another the 790. Suggest a video card or other means to connect the second display.
Post a picture of the the back of your PC so I can see what ports are available.
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790 sff has a vga and dp connection, you can use a dp to dvi converter or a pci-e dvi adapter ( it will use the onboard videocard)
If you want to go down the video card route, Dell suggests this one in the manual
Get the 2xVGA cable option (as pictured)-
They are pretty expensive though (~80)
512MB AMD RADEON™ HD6350 MT DT SFF
Bus Type (example integrated or PCIe x16) PCIEx16
GPU core clock 650Mhz
Frame Buffer Memory (onboard and shared) Size and Speed 800Mhz
Maximum power consumption 20W
Overlay Planes Yes
Maximum Color Depth 32-bit
Maximum Vertical Refresh Rate 85Hz
Multiple Display Support Yes
Operating Systems Graphics/ Video API Support DX11 and OpenGL3
Supported Resolutions and Max Refresh Rates (Hz) (Note: Analog and/or digital)
DVI Max : 1920x1200/32bpp @ 75Hz
VGA Max: 1920x1440/32bpp @ 75Hz
Min : 640x480/8bpp @ 60Hz
External connectors 1 DMS59 (DVI x2 or VGA x2)