are you in kde4?
in kde control center there is a menu setting all that
kcc/hardware/size and orientation, also in the multiple monitors setup(that remains disabled until you enable the second monitor first)
i have a netbook and tried to do it, but all i could was clone screen, i believe that the difference between the two screens i had(one 16:10 small with weird resolution and the other 4:3) and compiz didn't let me work in extended mode
i haven't used a nvidia card in a long time but i remember someone saying that the menu had a option to handle this settings too