Xubuntu/xfce4 resolution question

I've got xubuntu (with xfce4) running on one of my boxes. I have, in said box, an nVidia FX5600 AGP, connected to a widescreen LCD capable of 1680x1050 pixel resolution. But even if I specifically set the monitor to "generic 1680x1050 LCD" (my exact monitor type, a Syncmaster 2220WM, is not included in their monitor selection) I still don't get that as a choice in the desktop settings.

Can anyone tell me how to fix this?