I learned something when I upgraded my new Mac Pro with a 2 GB RAM kit, and this isn’t on any Apple documentation that I could find.
The Mac Pro handles RAM on two riser boards that project perpendicularly from the motherboard. Each riser board has four slots for RAM DIMM modules:
This picture shows one of the riser boards, populated with two DIMMs. The board slides into the Mac Pro case; in the picture, the two holes in the board are finger holes you use to pull the board out of its motherboard slot. It’s an excellent design (as is expanding the rest of the machine; I installed the two hard drives and the surplus optical drive stripped from my dead G5, and it was incredibly easy). By the way, the DIMMs in the picture look a bit odd and fat because each one runs hot enough to require its own heat sink.
The Mac Pro came with 1 GB of RAM, in two 512 MB DIMMs installed in slots 1 and 2 of the top riser card. The upgrade I bought was two 1 GB DIMMs (you must upgrade RAM in DIMM pairs). Apple’s RAM installation instructions told me to put them into slots 1 and 2 of the bottom riser card, which I did.
On reboot, the Finder reported only 2 GB of RAM, not the 3 GB I expected. System Profiler only recognized the two DIMMs in the bottom riser card. And there was a red LED lit on the top riser card, the one with the two 512 MB DIMMs. I knew that RAM was fine, as the machine had just booted from it with no problem.
So I shut down the Mac and swapped the two riser cards, guessing that the machine wanted to see the two larger DIMMS in the top riser card. And it worked. The Mac now sees all the RAM. Something to keep in mind if you’re planning to upgrade RAM in a Mac Pro.