Inspiron 8500 ATI Radeon 9000 AGP card won't support 1680x1050 on external monitor

I have an Inspiron 8500 15.4" WUXGA widescreen laptop and the Mobility Radeon 9000 32 MB card supports 1680 x 1050 resolution. I recently bought a 22" widescreen external LCD monitor that supports the same resolution as my laptop. But when I switch my diplay from my laptop monitor to my external LCD DVI connection), the resolution automatically scales down to 1280 x 1024. I have tried fiddling with the display controls, updating the graphics driver to no avail. Is there a way to get the graphics card to support a higher resolution for the external monitor. The external monitor clearly can handle this resolution since I have been able to use my wife's Thinkpad with this higher resolution. My suspicion is that the graphics card probably needs a tweak, and I would really appreciate any help in this regard.

 

Thanks