Also, to answer your question: Yes, you can dual monitors that are two different sizes. In the nvidia control panel you can "move" the monitors around to how they are on your desk. That way the edges will actually line up and your cursor will make sense.
It depends on the game. The second screen will either be black or will show your desktop. So, the game just runs full screen on whichever monitor you've designated as your primary monitor. The mouse is trapped to that monitor (except on some older games) and you would alt-tab to jump back to your full desktop.