external monitor on macbook pro

hi everybody!i set up a tripple boot machine. but i think i made some mistakes with the filesystems. ok it is learning by doing i guess...now i have a problem with my external monitor. i installed the ati-driver somehow and on the laptopscreen the native resolution is working 1440*900. but i can't get the external monitor to work. when i select "Big Desktop horizontal" it states that i have to restart the xserver. i did this with ctrl-alt-backspace. but when i log in again nothing changed... same after rebooting.thank you for your comments!greg