As they were shipped from the factory, G5 iMacs only support display mirroring. The LCD will show the same thing as the iMac's screen.

The video cards in them support display spanning, Apple just turned the function off. You can get a third party hack than enables it here:

http://macparts.de/ibook/

Disclaimer: I don't know if it is safe. It's not supported by Apple, so if it kills your computer, you may be out of luck.

Also, some of the very last G5 iMacs don't need the hack. Apple started supporting display spanning in them starting about the time the Intel iMacs came out. All you need with them is the proper adapter (VGA or DVI) to go to your monitor.