Can you open up your /etc/X11/xorg.conf file, and post it up. This will help to determine the settings required in the xorg.conf file to correct the resolution.
The problem could be your driver. I think the x1300 needs the ati official driver (will also work with vesa). If you are using the open source drivers, the card will not work properly as they are not compatible.
It could also be that the maximum resolution is set at 800x600. Changing this may resolve the problem. Some minor changes to the config file will be needed though.
The resolution section looks something like this (this is my one):
Modes "1280x1024" "1024x768" "800x600" "640x480"
Xorg uses the highest resolution, so if the highest resolution is 800x600, this one will be used.