I'm looking at upgrading the HD in my new Aluminum MacBook and have two options. Cost and impact on battery life are not deciding factors. #1. Speed ----- 320GB, 7200 RPM #2. Capacity -- 500GB, 5400 RPM Which would you choose? Capacity difference is obvious. How noticeable would the speed difference be between the two? As I understand it, the increased size of the platters on the 500GB drive provides a bit of a speed boost to a comparably sized 5400 RPM drive. 320GB is still a lot of space.