There is no point changing "ati" to "fglrx" in xorg.conf if the closed-source FGLRX driver is not installed on your machine. The FGLRX driver package is called ati-drivers and is not part of the package list on the LXDE LiveCD (see http://na.mirror.garr.it/mirrors/sabayo ... so.pkglist
) so, unless you installed it using the Entropy package manager after you installed SL 6 LXDE to your HDD, it isn't installed. Therefore you either need to install the package ati-drivers or revert xorg.conf to how it was and try the procedure in thread Can't get my radeon driver to work..
to see if you can get the open-source Radeon driver to work on your particular hardware.
EDIT (September 1, 2011 16:23 BST): Actually, before you try the method given in the above-mentioned thread, check on the AMD Web site if your ATI GPU is supported by the closed-source FGLRX driver and, if it is, study the thread Re: ATI Driver Installation
from last year and try the steps in that to see if you can get the closed-source FGLRX driver working. You'll need to replace the old version numbers in it with the current ones, which you can check using the package manager or via the on-line package search page (http://packages.sabayon.org/