You can change the screen saver settings in KDE to do what you want. I'm using OpenSuse 10.3 not Ubuntu, so yours might be a little different. Screen saver settings in KDE's menu are usually in Settings ->Appearance and Themes->Screen Saver.
Just set the time in minutes when you want the saver to come up, and set the lock mode. Example: 15 for screen saver and 20 for lock, or what ever you want.