Leon: No LCD can produce a higher resolution image than native on the screen. That is by nature because the pixels in the screen (as opposed to CRTs) are numbered and defined. So when you have 1680x1050 actual pixels on the screen, you cannot display any more. It is physically impossible. If you force a higher resolution with your graphics card on the LCD in Windows, the desktop will just become too big for the screen to fit in and you will just "float" around it with your cursor, not being able to see it obviously in one go.
namredel: Your monitor is specified for 1920x1080 indeed, so you should be able to force that resolution to your monitor. You do not need new drivers for your monitor. It seems like a driver issue. Did you properly uninstall Nvidia's drivers before you installed the ATI card?