You'll have to edit your /etc/X11/XF86Config-4 file and add the following entries:

Search for the ModeLine settings in the "Monitor" section and add the following line:
ModeLine "1200x800" 90.00 1200 1232 1384 1520 800 803 806 827
You might have to adjust the settings somewhat (esp. the 90.00 frequency might be
too high or too low - depends on your display characteristics; for an explanation see
man XF86Config-4 or google around a little)

Further, in the "Screen" section, "Display" subsection add the newly defined mode to
the list of existing modes:
Modes "1200x800" "1024x768" "800x600" "640x480"
The first mode is the default mode used for your display.

You can use the "xvidtune" utility to fine tune the settings.

Ciao,
Thomas