After installation of Linux Mint 18.1 "Serena" - Xfce (64-bit) on a Dell
  PowerEdge SC430 Server with an XGI Volari Z7 Graphics Controller
  there was only a single, disappointingly low screen resolution of just
  640 x 480 available, although resolutions up to 1600 x 1200
  are working.on Windows

  The terminal command inxi -Gx shows - the vesa driver wasn't loaded!

  -----------------------------------------------------------------------------------
  Card: XGI (eXtreme Graphics Innovation) Z7/Z9 (XG20 core)
  bus-ID: 05:07.0
  Display Server: X.Org 1.18.4 drivers: fbdev (unloaded: vesa)
  Resolution: 640x480@73.00hz
  GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.8, 128 bits)
  GLX Version: 3.0 Mesa 11.2.0 Direct Rendering: Yes
  -----------------------------------------------------------------------------------

  This was in the end the inspiration to explicitly load the vesa driver,,
  with the result that resolutions up to 1280 x 1024 are possible now.

  Just save the file xorg.conf to a temporary directory (e.g., download),
  adapt the bus-ID PCI:5:7:0 if necessary (no leading zeros) and copy
  it with sudo cp xorg.conf /etc/X11/xorg.conf to the /etc/X11/ directory.

  Perhaps also a solution for other Linux Mint and Ubuntu versions or for
  PCs from other suppliers.