What's the best way to upgrade ram? Specifically, I want to have at least 1GB in a MacBook I'm about to buy for my wife, and of course, it's cheaper to buy the RAM separately than buy it already installed. If I buy a 512mb MacBook (2 x 256MB), can I buy a single 1GB chip, or must I buy 512MB X 2? Do both slots have to be filled, and/or do both need the same size chips installed for the best performance? Thanks in advance.