I've scoured the internet and haven't come across a definite answer yet for why I would not be able to put 2x2GB ram modules into my Rev A G5 iMac. I've looked at the developer architecture guide from apple and it says nothing as to why a larger module won't work. (http://developer.apple.com/document.../apple_ref/doc/uid/TP40002046-CH206-TPXREF105) I guess I am just looking to understand where the limitation exists, is it hardware not being able to address the extra RAM or something else. I've also been unable to dig up much more info on the U3L memory bus to see if thats where the problem might be. I'm thinking that at the time 2gb modules were uncommon or not made so there was no reason to include them, but now its only alittle more than the 1gb ones. Anyone have any advice or direction for a place to go to find more info?