I'm not sure if the MGA driver is xrandr 1.2 compliant but, if it is, you can try creating and adding a mode to the monitor.
Code:
xrandr --newmode "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync
xrandr --addmode DVI-0 "1280x1024_60.00"
xrandr --output DVI-0 --mode "1280x1024_60.00"
The modeline was generated with cvt. You can try the same for whatever resolution you want. The output name varies from driver to driver, but you can confirm what the output name is by simply running 'xrandr'.
Adam