Apple has a cool little $19 adapter for the G5 that uses the open DVI port (if you are using an ADC display). I picked one up today and it is a short 2 inch DVI cable with a little white box at the end that has both a composite and S-video out jack. When hooked up it you can use it to mirror or have multiple desktops.
It has options for either outputting in NTSC (800x600, 1024x768 @ 60Hz) or PAL(800x600, 1024x768 @ 50Hz). It is well worth the money and is plug and play (like I had to say that right?).
You can find it at the Apple store under Apple accessories and is called the "DVI to Video Adapter". It looks similar to the other ones they have for the laptops but I haven't seen Apple offer this on tower before.
-Jerry C.