Here is a manual to enable a second monitor on Ubuntu using an ATI video card. First you need to check if you have the ATI drivers installed, go to System -> Administration -> Hardware Drivers. If the ATI proprietary FGLRX graphics driver is not installed select it and click on the Activate button. When the driver is installed you need to reboot your system.
With the ATI Catalyst Control Center enabled you can connect your second screen before starting your system. If all goes well the screen will automaticly be cloned and shown on both your screens from the moment you login. If you wish to enable/disbale or to adjust settings to either one of the screens go to System -> Preferences -> ATI Catalyst Control Center (Administrative).
Configuration update message, mark Do not show me this again and click the OK button
Multi-Display Tab, here you can enable/disable the selected screen or the way it’s cloned.
Display Properties, to change resolution and refresh rate settings