I just got my iMac G5, and when I bought it I assumed I could just add any old compatible memory and it wouldn't affect performance. However, it appears that you need two DIMMs of the same composition to get a 128-bit memory bus instead of a 64-bit memory bus. According to macbidouille.com, this can yield up to a 38% increase in the performance of Xbench memory tests. Many people have bought RAM of the same size but different composition, and they are getting a 64-bit bus. To dispaly the RAM bus size, run the following command:
ioreg -p IODeviceTree -n memory -S | grep ram-
If the ram-layout-architecture value is 1, then you have a 128-bit bus; if it is 2, you have a 64-bit bus.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2004100613102481