Try logging in as root from the command-line. Then (sorry, not very familiar with RH), try to run "system-config-display --reconfig"
The /etc/X11/xorg.conf (or XF86config) file probably has an incorrect entry for your screen. This will walk you through some settings, and you can try out different things.
I'd suggest trying some lowest-common-denominator values, like 800x600 resolution at 16 bit color, with the VESA or VGA drivers, just to see if you can get X going. From there, the GUI based tools are easier to manage, and will let you test different resolutions, before "committing" them to the config file.
It wouldn't hurt to have the manual for your monitor, too, because the config tool will probably prompt you for hsync and vsync parameters, and refresh rates.