I recently got an extra 22-inch LG monitor for my main PC to accompany the 27-inch main monitor. I was a bit uncertain about the settings under GNU/Linux, especially since I have Nvidia GTX 470 card.
If you have Nvidia drivers installed then you already have nvidia-settings installed. Open Konsole and become root and then open nvidia-settings. On the settings window enable the 'second' monitor, which will otherwise be 'disabled'. There are two options TwinView and Separate X. I will prefer TwinView as it allows you to move windows from one monitor to another, it's simply an extension of the primary monitor. Choose which monitor you want to set as primary display, this will be the monitor on which you will see the menus/panels and the desktop.
Once done, click on apply and then 'Save to X Configuration File'. This is where you might get stuck as there is no location listed there.