After you edit the file you need to restart X, I would go into runlevel 3 and try "startx" before making any permanent changes. If you added the modes line at the appropriate place (i.e. at the display subsection with the default color depth) I think it should work. LCD screens usually work great at only one resolution, so I don't think it's necessary to specify more than one.
So for a widescreen the section might look something like this (replace the resolution with your own):
If your default depth is 32, then I think that will work the next time the X server is started.