Hmmm... this is strange. The last time I checked screen configutation was in yast and yast2. Also (if you are using kde) there should be an icon on the LHS bottom to change resolution.
Anyway, the main file is /etc/X11/xorg.conf. You can make changes there and restart gdm/kdm. Then there is my favourate 'X -config' but then for some reasons it works perfectly for my computer.
Is the video card being detected properly? The xorg.conf will tell you that too. Make sure that you have a backup of the file before you make any changes to it.
Hope this helps.