I use Nvidia driver 97.73 with my M200 at 1920x1200. You should take a look at the modified inf file to see if it lists 1920x1200 as a valid resolution for the driver you are using. If not, you might have to add the value by hand and then remove/reload the driver.
I don't use dual monitor so I don't know if that configuration is the source of your problem. Try running in single monitor mode with the external display to see if you can get to 1920x1200.