Hey guys, new to the board, but long time follower. I have a question that has been bothering me for almost a year. I have been able to install 2GB sticks (up to 8 for a total of 16GB) of PC2-6400F from random servers (HP, IBM, etc.) in a 2008 Mac Pro without a single issue. But when I try to use 4GB sticks (also from random HP or IBM servers) of PC2-6400F, they are never recognized. All 4 of the red lights are on for each of the modules and the power LED blinks. No startup at all. I have tried every memory configuration possible with no results. I have tried over a dozen different working pairs of matching 4GB sticks on 4 different 2008 Mac Pro's without any success. Can anyone shed some light on this matter? I have searched and searched, but couldn't find anyone that has an issue like mine.