Are you sure you have an i810 video card? RHEL5.1 is a reasonable modern distro and the i810 is antique
Try to add a monitor section to your xorg.conf. Find the specs for your monitor and modify the HorizSync and VertRefresh accordingly. In the spec, the former is specified in kHz, the latter in Hz.
Add the line in red italic to the screen section
Viewport 0 0
Modes "1280x1024" "1024x768" "800x600" "640x480"
If you want 1024x768 as the default, remove the 1280x1024 or move it to the end of the line