I'm planning to buy a 13" Sandy Bridge Air, and am wondering about the tradeoff between CPU speed and battery life. What effect will a CPU upgrade have on both performance and battery life? i.e. Does a 10% bump in CPU speed correspond with a 10% increase in performance and a 10% decrease in battery life? I will be using the notebook in law school, primarily for note-taking, web browsing and movies. I rarely play games or edit video, although I occasionally edit photos. Thanks!