To edit the /etc/X11/xorg.conf file you will need a command line text editor like vi, vim or emacs, at least one of these will be installed on your distro. Then just use either "ati" or "radeon", just look for text in the /etc/X11/xorg.conf file that is similar to the one I posted in the earlier post.
An example, change:
This will tell XServer to load the driver for an ati card, this driver is already part of the linux kernel, thanks to the hard work of the distro developers, and of course the kernel developers and supporters.
Try this with "ati", then try "radeon", if this doesn't work, then try "vesa".