[ubuntu] problem with installing ati driver !

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http://ubuntuforums.org – hi i am new to ubuntu few days ago i downloaded ati driver 12.2 from "amd website" , and installed it . but after reboot this message shows up : "ubuntu runs in low-graphic mode ... " so i fix it with great effort (via recovery mode) , after that i used "Additionals Drivers" in "system setting" to install ati driver ! but it didnt work again ! i have no idea what to do now! my graphic is: ati radeon mobility 4550 please help :) (Hardware)