xorg.conf, or videocard, or ....

[Endymion]

Try booting into run level 3 (at the bootloader press any key, then 
select the kernel and then edit the boot commands, remove rhgb and put 
the number 3).  Log in as root and then run "Xorg -configure".  That 
should generate a fresh xorg.conf file based on your hardware and will 
probe the monitor for the right settings.  Just move the new xorg.conf 
file to /etc/X11/xorg.conf (be sure to back up the old one just in 
case).  Hopefully this will fix everything for ya.

-Endy