How to switch to a particular resolution? Xorg complains "No valid modes for "DFP-1:2560x1600"; removing." and ignores my setting.

[Yuri]

Xorg for me starts with the resolution 1920x1080.

I know that 2560x1600 is supported because the NVidia settings program 
can switch to 2560x1600 by typing 2560x1600 into it. The window manager 
stays in a square with the size 1920x1080, and the extra space appears 
around it.


But how to change xorg.conf in order to make 2560x1600 a default resolution?


I tried to add the line Modes into this section:

         SubSection "Display"
                 Viewport   0 0
                 Depth     1
                 Modes      "2560x1600" "1920x1080"
         EndSubSection

But Xorg fails to accept the 2560x1600 value with this warning:

[  9995.669] (WW) NVIDIA(0): No valid modes for "DFP-1:2560x1600"; removing.

and still uses 1920x1080.


Thank you,

Yuri