Windows XP can handle up to 7 monitors, I believe. I've got an extra video card in both my work and home systems.
When you right-click on the desktop and pull up properties, the "settings" tab will show both monitor (cards) you have in your box. Each window is numbered and you can position them in a virtual desktop. By tweaking the position, you can align things up very nicely.
Note the"Use this device as the primary" and "Extend my Windows" options when I have the second monitor selected:
