I have installed xf86-video-ati and lib32-ati-dri packages, system starts, but I need to improve performance of 3D games and some KDE effects.
After installing catalyst package, setting nomodeset for kernel and blacklisting radeon, X isn't starting, probably because one of my graphics card (integrated HD4250) is no longer supported in latest releases. Last version that was working was 12.6 and catalyst owns >12.10 already.
I would try myself, but I must rely on your knowledge, because getting X working back after such failure is painfull to fix, had to do it before .
You have to downgrade to Xorg 1.12 by adding
[xorg112] Server = http://catalyst.apocalypsus.net/repo/xorg112/$arch
above all other repositories in your /etc/pacman.conf.
Then run pacman -Syuu (double u does the downgrading)
and add the [catalyst-hd234k] repository as described in the wiki.