Have you installed 915resolution ?
# yum info 915resolution.i386
Loading "installonlyn" plugin
Name : 915resolution
Arch : i386
Size : 16 k
Repo : fedora
Summary: Intel video BIOS hack to support certain resolutions
915resolution is a tool to modify the video BIOS of the 800 and 900 series
Intel graphics chipsets. This includes the 845G, 855G, and 865G chipsets, as
well as 915G, 915GM, and 945G chipsets. This modification is necessary to
allow the display of certain graphics resolutions for an Xorg or XFree86
915resolution's modifications of the BIOS are transient. There is no risk of
permanent modification of the BIOS. This also means that 915resolution must be
run every time the computer boots inorder for it's changes to take effect.
915resolution is derived from the tool 855resolution. However, the code
differs substantially. 915resolution's code base is much simpler.
915resolution also allows the modification of bits per pixel.
yum install 915resolution.