Do a news search on rage3d for custom resolutions or something.
In a nutshell, find in the registry the value "DALNonStandardModesBCD1" for the Radeon and modify it by adding the data "1280960" at the end of what is already there. Then delete the value "DALRestrictedModesBCD1" since its default data restricts that resolution.
Does anyone know why ATI does this? Do other chip makers do the same?