Very good post all. Good info.
Just to share, I recently try using only image ads for my top of page ads. You know what, I have earned more clicks and earnings. Surprisingly, there was a click that earned me 2.53 !! But there were also clicks that valued at 0.01
I already try this for a week, the results, I am quite happy with the number of clicks and earnings. Will try for another week before I decide though.
From my experience with adsense, the large rectangle generates the most clicks and earnings. Its best to use the large rectangle near the top of your page and in between your content.