First I would try pressing <ctrl><alt>+ (or -)
*note use your number pad's + and - keys.
If that doesn't work try and find your XF86Config file. Mine is located at:
open it up in a text editor and look for line similar to:
Modes "1280x1024" "1024x768" "800x600" "640x480"
Make a copy of the file first (You can always delete it and revert back to the old one albeit "broken"):
cp XF86Config-4 XF86Config-4.backup
Then place what resolutions your setup permits as shown above.
Also make sure the Horizontal Sync and Vertical Refresh are correct. You'll probably have to check the manual that came with your monitor.
...but definitely don't use *mine*. You risk frying your monitor with the wrong settings.
Hope that helps.
ps. When I first started I used this
then moved on to this