I use an xorg.conf, and have the following;

Section "Files"
        ModulePath "/usr/lib64/xorg/modules"
        ModulePath "/usr/lib64/opengl/xorg-x11/extensions/"
EndSection

Your xorg cant find the dri and dri2 modules because its not looking in the
second directory.

Also, it might be worth trying the latest driver, so
# echo "x11-drivers/ati-drivers ~amd64" >> /etc/portage/package.keywords
Then emerge ati-drivers again.

If that doesnt help, send the X11 log again after you've made those changes.