In my opinion, modelines are just a workaround for bad writen or missing EDID informations concerning a particular resolution. For other resolutions, not covered by the modelines, I guess it is the EDID informations that is used. I don't have another explanation for your black line.
I haven't tested those Dell drivers because I don't have too much time to spend. But I did look in the package and I didn't noticed what could make the difference. Maybe someone who tested them may tell more. For the Twinview thing, you got it right: you must have a second monitor to have the laptop work.
Hardware: Dell Inspiron 2650, P4M @ 1.8 GHz, chipset i845, 512 MB RAM, nVidia Geforce 2 Go with 16 MB RAM, 20 GB HDD.
Software: BIOS A10 (3.11.01.52.AB video BIOS), Fedora Core 4, Kernel 2.6.12-1.1398_FC4, Xorg 6.8.2, NVIDIA 7667 drivers.