I am curious as to what enables the new Macbook Pro to support 4 gb versus the 2 gb of the old Macbook Pro. I clearly don't know very much about Ram and I don't yet need 4 gb but I want to understand why there is a physical limitation. The Mac Pro is said to hold a max of 16 gb of RAM on apple's site but it can actually hold 32 gb. How is this technology different? Are there ways to upgrade an older Macbook Pro to hold and use 4gb? Thanks for you help!