By now you really should be using Fedora Core 2! But here's the work around for you, if it is as I suspect an issue that Fedora Core 1 had with detecting some monitors:
From the main login screen select session and "Failsafe". Then log in as root. Now at the command line issue the following command:
This will drop you down to a true command line, outside of the X server which renders the GUI. Now you can use the DDCprobe tool, to detect your monitors refresh settings. The output of which should be noted down:
We need to restore the runtime now to one with a gui so issue the following command:
Logt back in as your normal username, open a terminal and become super user:
Now we're going to input the refresh settings which we've found into the XFree86Config file which controlls the resolution of your display. Before we do anything however we must make a backup copy of the config file just in case anything goes wrong.
cp XFree86Config XFree86Config.backup
Now we want to edit the config file, so again from the command line enter the following command which will launch the text editor gedit.
gedit ./XFree86Config &
We're going to want to edit two sections of the config file in order to get the display working at the resolution you want. The first section we have to modify is entitled the "Monitor"
section, in this section you'll want to modify the refresh rates for the screen with the output found in the results from the ddcprobe. The section your looking for will look something like this, edit the lines marked "(EDIT THIS LINE)":
VendorName "Monitor Vendor"
ModelName "Monitor Model"
HorizSync 31.5 - 48.5 (EDIT THIS LINE)
VertRefresh 50 - 70 (EDIT THIS LINE)
Next you'll want to input the resolutions you want the screen to display. These must be input in the order in which you wish to use them, so if you input 1024x 768 and also list 800 x 600. The X server will try to use the 1024 setting first. Bare this in mind when choosing the resolution list.
Find the section entitled "Screen" (normally found at the bottom of the config file) and edit the following lines, so they look like this:
Modes "1024x768" "800x600" "640x480"
Save the file and restart the X server by pressing Ctrl + Alt + Backspace. This should change the resolution to 1024 x 768. If nothing appears, or it hasn't worked. Post back - and I'll talk you through restoring your old XFree86Config file. Although I doubt that will happen!
Good Luck! (But i do implore you to look into Fedora Core 2, its much nicer)