Game not detecting correct resolutions

[Neurorelay]

I just picked up Roller Coaster Tycoon 3 and it only shows 1024x768 and 800x600 as the available resolutions. Now in games like counter strike I would just manually configure it in the ini file. But some games don't have this, so how would I get the game to use the resolutions I know my 6800 GT can handle.