Are you using Gnome? If so, navigate to System-->Administration-->Display. Under the hardware tab, click on the "configure" button next to Monitor Type.
Scroll down to Dell and you model will be there. Make sure that you pick the [b]exact monitor model and type (digital or analog) that you have. Your monitor is listed as a selection.
Then naviagte back to the "settings" tab and see if your resolution comes up as supported and that you can choose the one that you want.
See if it works.