In short the problem is like this. There are lots of common resolutions like 640x480, 800x600, 1024x768 and so on. The resolution you want to use is not a standard resolution. On windows this resolution is added by a "modeline" which I think is a .inf file (or it is autodetected).
To get this resolution on Linux you need to create a modeline called "1920x1200" and put it in the monitor section of the monitor and then you can just use it. Calculate a modeline using http://www.sh.nu/nvidia/gtf.php
(based on http://gtf.sf.net
). The modeline it outputs is this one:
# 1920x1200 @ 60.00 Hz (GTF) hsync: 74.52 kHz; pclk: 193.16 MHz
Modeline "1920x1200_60.00" 193.16 1920 2048 2256 2592 1200 1201 1204 1242 -HSync +Vsync
In this case because the mode is called 1920x1200_60.00 you need to use this name instead of plain 1920x1200. You can rename it ofcourse.