this may be a shot in the dark, but here goes...
In the hardware setup for SUse (i think it may be in YAST modules), go the the Hardware->Graphics Adapter setting. You'll notice you have a section for Monitor and another for Resolution. You can setup you monitor in the monitor sections (as the name suggests). This will set the hard limits for what the capability of your monitor is. Once this is set, THEN to the resolution setting. The only thing i can think of is you are trying a resolution that you have not marked as legitimate for your monitor.
Sorry if this is stupid, but its all i could think of right off hand..
worth a shot