Are you using kde or kde4? From your description, I'm not certain if you are having a video problem or a KDE problem.
Read through your xorg log. Look at lines that start with "(EE)". Those indicate errors. If X can't use any of the sizes in your configuration for some reason, it should be indicated there.
To see if it is a KDE problem, you might try creating a new user and log in as that user. That could indicate a problem with kde. Also, use a different desktop such as xfce4. If the both of these work you may need to delete your ~/.kde or ~/.kde4 directory and log out and log in again.
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