Just got a mac pro 2.66GHz. It came with 2GB in a 4x512 configuration. I want to add an additional 4GB of RAM. Should I purchase 4x1GB or should I purchase 2x2GB? I am being a little nearsighted in my purchasing as I believe this is what I will live with for awhile so I am wanting to make the configuration to get the best performance for the foreseeable future working with the current 2GB with the addition of the 4GB's. Your thoughts?