How do you change the resolution to 1920x1200?

I'm assuming that you're using either Windows Vista, or XP, and that your graphics car does support 1920x1200. You should double check to make sure that your card can support the higher resolution.

You should go to your desktop, find a blank space, and right click. Somewhere on the context menu, there should be an item that says either "personalize", or "properties." If you're using XP, it will pop up a menu, and you should browse through the tabs to find the one that controls the resolution. This will probably be in the appearance tab.

On vista, you will be taken into a control panel sub-menu. Click on Display Settings, and it will pull up a window that will allow you to alter your resolution.

If neither of these methods allow you to increase the resolution, and you're certain that your card supports the higher resolution, then you will need to get the new monitor. The operating system (or graphics drivers) will automatically pick up the new display settings once you install the larger screen, and the option should become available. If it still does not work, search for display drivers that correspond with your model of monitor.