Ethernet and Coffee

First use gtf x y refresh to get the Modeline:

gvolk@wumpus:~$ /usr/bin/gtf 1680 1050 60

  # 1680x1050 @ 60.00 Hz (GTF) hsync: 65.22 kHz; pclk: 147.14 MHz
  Modeline "1680x1050_60.00"  147.14  1680 1784 1968 2256  1050 1051 1054 1087  -HSync +Vsync

gvolk@wumpus:~$

Then take the Modeline output and put together some xrandr command lines like these…

xrandr --newmode "1680x1050_60.00" 147.14  1680 1784 1968 2256  1050 1051 1054 1087  -HSync +Vsync
xrandr --addmode VGA1 1680x1050_60.00
xrandr --output VGA1 --mode 1680x1050_60.00

When you are happy with the results, put the three xrandr commands in ~.xprofile and the resolution will be set every time that user logs in.