I bought the Quad, its truly a beast in a box. If your a audio or video editor, then i would say yes, but thats my suggestion.
Example, I use Pro Logic 7.1, by using this software on my Quad, im able to process more audio data, while im recording and listening to the audio.
Depends on what you are going to use the system for. If basic stuff like listening to music, internet search, emailing etc, i would get something like imac g5, or PowerMac 2.0Ghz (doesn't have to be dual).
Or if your doing some serious editing video/audio i would get the PowerMac G5 2.7Ghz, or Quad