Section "Monitor"
Identifier "My Monitor"
# HorizSync is in kHz unless units are specified.
# HorizSync may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.
HorizSync 31.5 - 84.0
# HorizSync 30-64 # multisync
# HorizSync 31.5, 35.2 # multiple fixed sync frequencies
# HorizSync 15-25, 30-50 # multiple ranges of sync frequencies
# VertRefresh is in Hz unless units are specified.
# VertRefresh may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.
VertRefresh 56-76
Modeline "1440x900_60.00" 106.47 1440 1520 1672 1904 900 901 904 932 -HSync +Vsync
EndSection
in xorg.conf-vesa, and X does not seem to run if I rename it xorg.conf...
Yes, you need the "Screen" part too. Put it in the same file. But your mode is named "1440x900_60.00", not "1440x900".
Another way to do it, without messing with xorg.conf is to edit your /etc/gdm/Init/Default and add the xrandr lines you used at the end, before the exit command. Or if you already have the Monitor section in your xorg.conf.d, you can just add the last xrandr command in it. Several ways to do it actually.