Sorry for the delay in replying, I have just returned from hernia surgery
Anyone who has problems selecting a resolution better than 800x600 needs to add the following lines to the Monitor section of their xorg.conf file, TAKE A BACKUP OF THE FILE FIRST
After altering the file save it and restart X using CNTL-ALT-BACKSPACE and you should find a lot more options when you set the resolution. Try settings one at a time and if you do something stupid (been there, done that) just wait and the system will jump back to the previous setting.
Why does this happen? I think that there is a bug in the monitor detection part of Ubuntu/Mint setup, if it doesn't recognize your monitor it assumes that it is an SVGA device with maximum resolution of 800x600. Things could be worse, how many of us have installed MS Windows and been met with a 640x480 screen in 16 colors?
P M Gant