I currently have ATI Radeon x800xl (pci express) with a 19inch Samsung syncmaster 940b. I cannot get the desktop (or video game) resolution to go bigger then 1280x960. The desktop "can" get bigger but it makes the image much larger then the screen. The max resolution says 19xx X 14xx
Just because your graphics card can go up to a certain resolution does not mean your monitor will be able to.