I would say a Power Mac G5 no, not for a long while yet.
They were never "that" rare. They were manufactured for a good 3 years before being replaced by the Mac Pro in summer 2006. They will still be a hobbyists thing, and worthy being part of a collection, but I doubt they would be worth anything substantial for the next 15-20 years, as long as they are kept it top notch condition.
Apple collectors items You have to look at stuff like the TAM, G4 Cube, Power CD, Pippin, stuff that had a very short life, when released.