What is output by `xrandr --verbose`? If EDID information is
being reported properly, it should show you the possible
resolutions for your monitor (presumably containing
1680x1050, since that's what you're expecting.) if not,
It may be that your EDID information is goofed and you need
to tell Xorg to augment that information with more resolutions
by adding modelines to your Xorg.conf (or however the hell it's
supposed to be done now)