Open your xorg.conf file as root typing this in a terminal:
Look for the monitor section:
Here is mine:
# Auto-generated by mkxf86config
HorizSync 28.0 - 96.0
VertRefresh 50.0 - 75.0
Now change the H and V sync rates so the fit to your screen. Usually you will find the info in the manual or on their website.
Then scroll down untill you see the Section "Screen". Change the resolutions to your needs, remember to check out the default depth and change the resolution that corresponds to this,
After you typed in the new values press Ctrl + Q. Then Y and <enter> to save the values.
Now just restart X Ctrl + Alt + backspace
Hope this helps you out.