Re: 2kx2k and 1600x1200 the same card
Try adding the following to the screen section.
Option "TwinViewXineramaInfoOrder" "DFP-0"
Alternatively you can run nvidia-settings and under "X Server Display Configuration"
select the monitor you want primary , check the "Make this the primary display for X screeen"
and click apply. Then click "Save to X Configuration file". Use the save xorg.conf file.