Dual screen

Hi guys,
I have a laptop + another monitor. I have ATI drivers installed, and the catalyst control center works. I can extend my monitor, but I cannot higher the resolution of my second screen (it’s 1600x1200 - but my laptop is only 1280X800) - so I’m stuck with a 2560x800 ‘total screen’. How can I get my second monitor to have it’s native (big) resolution?