Adding RAM to 1.6 GHz G5

Question

I just finished a rather strange process of adding RAM to my G5 which I thought I would share, in case it helps others:
First of all, I have a 1.6 GHz CPU machine, which has 4 DIMM slots. According to the Setup Guide, since there are only 4 slots (not 8), you need to use 333 MHz, PC2700 DIMMs (not 400 MHz, PC3200 DIMMs).
When I first got my G5, it came with 256 MB of RAM (2 x 128 MB 333 MHz, PC2700). About a year ago, I added 512 MB (2 x 256 MB 333 MHz, PC2700), bringing the total up to 768 MB. This addition went smoothly, without any problems.
Recently, I decided to replace the original 256 MB of RAM, with two more 256 MB modules, bringing the total up to 1.5 GB, so I ordered "1 GB DDR400 PC3200 2x512 DIMMS" (Part # M9653G/A) directly from Apple. Note that the RAM Apple currently sells is 400 MHz, PC3200, not 333 MHz, PC2700. In fact, before placing the order online, I called Apple to ask about this, and the person I talked to assured me that this would not be a problem, and that my 1.6 GHz would work with the 400 MHz, PC3200, despite what the manual says, so I ordered them.
Upon receipt, I removed the original pieces, installed the new ones, and my G5 would not acknowledge their presence. It said I only had 512 MB. I called Apple to report the problem, and they suspected it was simply faulty RAM, so they sent replacements. When I got them, I installed them, and got the same results. I called Apple to tell them it still wasn't working, and they had no idea why. Once again, I pointed out that they were sending me 400 MHz, PC3200, not 333 MHz, PC2700, and they still insisted that this was not the problem, but I started to suspect even more that it was, especially after trying to upgrade twice.
In order to test this theory, I ordered RAM from MacMall. This time I ordered 1 GB, in the form of 2 x 512 MB, 333 MHz, PC2700 (made by Kingston, Manufacturer Part No. KTA-G5333/1G). When I received these, I moved the original replacement RAM up into the top two slots, where the original ones were, and placed the two new ones in the bottom two slots. When I tried to turn the computer back on, it wouldn't. The light on the front would flash once, then pause, then flash once again, then pause, etc. When I checked the manual's troubleshooting section, it described what two, three, four, and 5 or 6 flashes meant, but not one. URGH!
At this point, I had no idea what was wrong, so I took my G5 to my local Apple store to ask them. The guy there had the idea of putting one of the original replacement RAM in one of the two top slots, and one of the new replacement RAM in the other top slot. Then he put one of the original ones in one of the two bottom slots, and one of the new ones in the other bottom slot. Believe it or not, not only did the computer turn back on, but it also recognized the new total of 1.5 GB of RAM!!!
This was such a strange process that I thought I would share it, in case someone is also currently going through this process, or wants to in the future.
Hope this helps, and I also hope I've made it clear exactly what happened. If anything isn't clear, please let me know, and I'll try again.
Trent
Power Mac G5   Mac OS X (10.4.4)  

Answer 1

PC3200 is fine for that system. The problem was that you were installing the RAM in the wrong slots.
The RAM must be installed in matched pairs, one in the top, one in the bottom, working out from the middlemost pair.
Viewed from the side
======== Slot J14
======== Slot J12
======== Slot J11
======== Slot J13
So, one pair of identical parts goes in J11/J12, and another pair goes in J13/J14. Doesn't matter if the bigger pair is first or second.
This wasn't in the manual for your Mac?
Quad G5 2.5Ghz 4.5GB 2x250G, PB 15" 1.5Ghz,80G,1.5G   Mac OS X (10.4.4)