I have an extra iMac G5 that I no longer use for compatibility & speed reasons, and saw that you can control an additional "virtual" display on an iPhone (such as iDisplay). Is there a piece of software out there that will add another "virtual" external display to my main Mac, so I can control that additional screen through VNC on my G5?
Are you looking for something like Screen Recycler? It allows you to use another Mac as a secondary display. If I understand your question correctly, this seems to be what you're looking for.