I had the same problems with my NVIDIA GTX 550 Ti. Blacklisting nouveau worked for me, I didn't try 'nomodeset'. By the way, what is the supposed way to install packages from extra immediately after installing and before booting the new system?
installpkg --root /mnt xf86-video-nouveau-blacklist-noarch-1
when the root partition was mounted under /mnt didn't work for me as expected. The entry in /var/log/packages tells me that the package is installed, but the blacklist file wasn't there. So I ended up by blacklisting nouveau from hand.