The ram should be matched in dimm 1 of each ram bank, meaning if you run matched pairs in dimm 1 & 2 of one ram bank, the machine will not work. Here's a visual that should help orient where your first matched pair of ram should be placed.
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Another way to think about it is when you are inside your powermac g5 with the fan removed, along the upper right hand corner of each ram slot, the slot will be numbered 1,2,3,4 etc. Stick the first matched pair of ram in the slots numbered one (there should be a single slot labeled 1 in each ram bank), slot 2 for the second matched pair etc. This is how you install ram into a Powermac g5. I hope this gets you up and running.
Good luck.
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Oh I just realized you said
"boards" in your previous post - as in
memory riser cards possibly? The name of your thread is confusing as it has parts of two (internally) very different machines. How many cdrom slots are on the front of the mac? One slot will be a Powermac G5 (PowerPC) which correlates to my initial instruction above. Two slots will be Classic Mac Pro (Intel) which are different machines. To install ram into cMP, follow these instructions.
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Use this instruction to ensure your ram is correctly installed before moving forward.
Good luck!