if i have MBA with i5 1.7 GHz and MBA with i5 1.8 GHz, is there any difference? can someone explain what that GHz does, i think what bigger it is, that better it is. Am i right? What does that GHz do, is it important? what about if i have i5 1.8 GHz and i5 2.5 GHz, is there a difference or 2.0 and 2.7 GHz? i5 and i7 is there some kind of big difference? help me !