Check out the "screen" section in the file cucu was talking about and check if '1024X768' is in there.
Code:
Section "Screen"
Identifier "Screen0"
Device "Intel 810"
Monitor "Monitor0"
DefaultDepth 16
Subsection "Display"
Depth 16
Modes "1024x768" "800x600" "640x480"
EndSubsection
I think redhat has a configuration program for it too.... type 'redhat' at the command line and press <tab> twice to see if there's anything that might help you.