I checked them, but its all set. The modeset=1 line is there and the radeon driver is installed( but the xorg now in operation uses the fglrx driver). (but there is also the ati driver... )
Look at the directory "/etc/modeprobe.d". There is a file "radeon-kms.conf" and it should contain the line "options radeon modeset=1".
You also need IMO the "radeon" driver for ATI cards.(xserver-xorg-video-radeon)