Hi, I currently have 6GB in my Mac Pro (4-core Xeon 3500 Series) 3rd Gen. Early 2009 and looking to upgrade to 16GB as I use it mostly for video editing. I am not to sure about which make of Ram to buy but I head the name Crucial pop up quite often. I did a scan using the software they provide and it gave me a message saying Memory Type: DDR3 PC3-8500, DDR3 (ECC) Maximum Memory: 16GB Currently Installed Memory: 6GB Total Memory Slots: 4 Available Memory Slots: -2 Your system may support multiple memory configurations based on the density, quantity and type of memory modules installed. Please consult the documentation for your specific system. To get to 16GB would I have to replace all my RAM and buy 12GB kit (4GBx3) ? I had a live Chat with Crucial and they said they did not understand the -2 slots message and it was best to speak to Apple. I have been on hold now for 30 mins trying to get an answer.