I understand that Powermac RAM has to be added to pairs of two, but do they have to all be the same size. My instinct would say no, but I just want to make sure. I searched, but there are way too many threads.
This is correct for the G5 PowerMacs. Memory must be installed in identical pairs (size and speed, preferrably from the same manufacturer, too) but you can add additional pairs that differ from the first pair (eg. 2x256MB + 2x512MB + 2x128MB if you want).
On the G4 PowerMacs memory can be installed in any combination (no pairing needed).