You could do what n0cmonkey said or you can do it my way. Just incase one of the ways does not work you have a 2nd option
Most distro's come with vim wich is a cli text editor. At the command line as root type vi /etc/X11/XF86Config or vi /etc/X11/xorg.conf
It all depends wich X system you are using.
Now you need the horizontal and vertical refresh rate specs for your monitor. You will look for these lines in the file.
HorizSync 30-95
VertRefresh 50-160
Put in your monitors specific refresh rate range. Then you want to scroll more toward the bottom of the file and look for this section.
Section "Screen"
Identifier "Screen 1"
Device "GFFX 5900se"
Monitor "Sceptre D77D"
DefaultDepth 24
Subsection "Display"
Depth 8
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x1024"
ViewPort 0 0
EndSubsection
If you have the DefaultDepth set to 24 it will use the section that is labled with the Depth 24. Set your resolution you want to use there. It only needs one set resolution and does not need them all listed.
With vi in order to edit the file just hit the insert key and start editing parts of the file that you would like, as per my examples above.
When you are done editing hit the esc key and then hit the shift+colon key at the same time. Then type wq to write and quit then hit the enter key and it should write and save the file. It should have dropped you back to cli at this point. Try getting into X at this point to see if it worked.
GL,
MNKyDeth