Need higher screen resolution

Dr. Creel, et. al.,

   First to Dr. Creel:  Thank you very much for PelicanHPC!

   My main question is: How do I boot up the Xfce Desktop at greater than 800x600?
   I'm using Matrox's Millenium 450 video card which I've used at many higher resolutions before.
   At the initial (menu) screen, (choosing the default option), and hitting the "Tab" key, I've tried adding
   each of these to the list of boot parameters:

   screen=1024x768
   screen=1280x1024
   screen=1600x1200
   xmodule=mga
   xmodule=svga
   vga=normal
   vga=771

   But there is no change. The Desktop is always at 800x600, and that is the maximum resolution
   listed in "Display Settings".

   I've even tried entering "sudo startx -- -dpi 100" instead of just "startx" when I want to get the GUI.

   Does anyone know what I should do?  Have I missed something simple/obvious?

                                                  Thanks in advance,

                                                  Mitch