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.