crazybill has a good point... if you have any visuals screen 
 
If you don't (or just wanna change it text based) edit /etc/X11/XFree86.conf
	Code:
	Section "Monitor"
    Identifier  "Samsung SyncMaster 753dfx"
    HorizSync   30.0 - 71.0
    VertRefresh 50.0 - 160.0
EndSection
 The Identifier isn't important.. it's freetext.
The HorizSync and VertRefresh is something you'll want to change (probably).  Doco on your monitor type should give you the values you want....
If you still have problems.. try running (as root in a console window): X -configure.  It should create a XFree86.conf file in the directory you're in (or maybe just in /root).  You can either run that file with the command it tells you to test with, or copy the file to /etc/X11 (overwriting the current XFree86.conf).
Good luck 
