Make sure your graphics card can run two monitors, if it has two monitor connections it should be fine. Once you connect it up, right click your desktop and got to properties. Under settings go advanced and you can set what each monitor does, whether you want it to duplicate or have windows extend on to it.
if your graphic card have 2 video output, its just plug&play--just connect the video cable to the graphic card, even when the system is running. WinXP and above with the current display driver can detect the present of 2nd monitor automatically.
or google multi/dual monitor for more details. one thing to take note when running multi-monitor on photoshop is the screen color calibration. even 2 monitor from the same model may have different color output.