I currently only have the 2gb that came with the Mac Pro. I'd like to add another 4gb for a total of 6. I've found two 2x2gb sets that appear to fit all the necessary specifications for around $40 on Amazon, from Nemix ram and Mitecs. Does anyone know anything about these brands? Does the company you buy the memory from make much difference? Is there anything else I need to consider? I've never done this before so I just want to make sure I don't forget anything. Thanks!