This is really strange...
At the beginning I installed Firewalk's guide and I came to this problem, I somehow managed to break my system and after a reformat I followed the guide again except now I can select any resolution and also there's no mouse tearing (!?)

I did exactly the same thing !
Anyway, thanks for all your help guys, maybe I didn't follow the guide well enough
http://forum.insanelymac.com/index.php?showtopic=69345&st=0