Well, if your screen supports it (are the 2001 and 2005 the only ones that do?). The 2005 has an option under the monitor's menu that you can change, or you can just use your video cards drivers.
For nVidia, right click the little nV icon in the taskbar/system tray and there should be an option to change it (90, 180, and 270 degrees). Set it to 90 and then turn the monitor sideways (it only will go one direction so don't break it trying to force it to go the other way).
For ATi, I don't know, but I would imagine that its not too much harder to find than with nVidia.