Yes, you can do this with Applescript or Automator.
The way I've done things like this is to use Automator to create a Service that is available in all applications. A service can be made to run a bash/zsh/ruby/whatever script. Then you can bind a key combination to the service in the Keyboard pane of system preferences.
Near the end of this MacBreak podcast this procedure is shown in detail.
Edit: I just noticed you're not using Snow Leopard. I don't think creating a system-wide service is as easy with 10.5.8. I guess you can't upgrade since it's a Powerbook. You might want to look into FastScripts which allows you to assign keyboard shortcuts to applescripts (free to use for up to 10 keyboard shortcuts) and use the following Applescript:
do shell script "/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine"