Error messages like this:
(II) NV(0): Mode "1600x1200" is larger than BIOS programmed panel size of 1280 x
1024. Removing.
can probably be solved by installing nvidia-glx and switching to the nvidia driver. Or by installing whatever gets you the nv driver, and using that one. (Mine was the first case.) The Dell doesn't have any place in the BIOS to program a panel size.
I also had:
Option "ExactModeTimingsDI" "TRUE"
Option "UseEdidDpi" "FALSE"
Option "ModeValidation" "NoEdidDFPMaxSizeCheck,NoVesaModes"
in /etc/X11/xorg.conf, but I don't know if those did any good. Got the hints from ubuntuforums.com.