Looks like the problem lies with my video card.
I'm on my girlfriend's Dell and it came bundled with an Intel card:
i810 (82845G/GL[Brookdale-G]/GE)
Here is what the Xorg.0.log tells me:
(II) I810(0): Not using mode "1680x1050@55" (no mode of this name)
(II) I810(0): Not using mode "1240x720" (no mode of this name)
(II) I810(0): Not using built-in mode "1600x1200" (width too large for virtual size)
Googling led me to the conclusion that this card relies on the BIOS to set its modes and evidently my BIOS does not have what I want:
http://www.mail-archive.com/devel@xf.../msg07809.html
There seems to be a way to become independant of the BIOS...
Peter