I am going to buy a new MacBook Pro and I am wondering why the 15" 2.66ghz is $450 more than the 13" 2.53ghz? The only differences are the .13ghz differnece, the 2" bigger screen, and the graphics card on the 15" also has a9600M GT with 256MB......Is this really worth the extra $450 or not? What are these three extra things going to do better than the 13"?