I have a Quad Core Mac Pro with 2 GB of RAM (4x512 MB) running Leopard. I am running into some memory issues with some larger Photoshop CS 3 files, and I want to upgrade. I faintly remember reading about maximum memory configurations, (i.e. the number of slots used) but I need to upgrade. If I wanted to add 2GB more, should I get another 4x512 MB? What if I wanted to add 4 GB more? 2x2 GB or 4 x 1 GB ? Help!!!!!