G5 256mb ram sticks not recognized

Probably a compatibility issue, either the type of ram or because of the size of the ram and the way the mac buss is addressing it.

If that's the PCI version it can only take a maximum of 4GB. The PCI-X version can take 8GB. If it has 4 RAM slots, it is the PCI version, if it has 8 slots, it's the PCI-X version. They both have the same model identifier number, PowerMac7,3.

I had a similar issue but after much trial and error realised the sticks had to be placed in certain slots (no doubt how the dual channel works) which I wrongly assumed was next to each other. Mine is a Late 2005 G5 DC 2.3.

I have two bank of 4 slots.
Each slot has a number (1-4 on both banks)
When adding memory in pairs they must be added as follows :-

Bank 1 Slot 1
Bank 2 Slot 1
Bank 1 Slot 2
Bank 2 Slot 2
etc etc

I added RAM recently by just putting them in the next two available slots and had less than before - took me a while with a torch and trial and error to work out what was happening...

Glad to hear you figured it out. I am having the same issue with mine. But keep an eye on it. I did manage to figure out how to get it to recognize the extra ram in my PM G5 but it lead to random crashes and random ram sticks failing during the boot diagnostic and the amount of ram the computer recognized kept changing. I think the fact that I am using several different brands and they are not all the same size even though they are matched pairs so I am still working things out on mine. I hope you don't run into the stability issues I did. I am back to it only recognizing 2gbs after some re-arranging but at least the computer is stable now.