ati generally does not have as many resolutions available as nvidia.
when i had a widescreen LCD a while back (it was a 17" phillips like 2 years ago) i HAD to buy an nvidia card because ati at the time did not even support 1280x768 ( 6 months ago their drivers didnt seem to support 1680x1050 either).
i remember like 4-5 yeras ago, i didnt want to use an ATI card becuase they did not support 1280x960 or 1152x864. you can probably use something like power strip or one of those video card tweaking utilities to set it to whatever you want though.