Have you checked the official Adsense heatmap? http://free-seo-helpdesk.blogspot.com/2008/11/adsense-placement-guide.html
2 250x250 next to eachother above the fold usually works well. Use the Search function to find more topics about this!
Well first of all I would suggest not to put Adsense ads ALL over your site, I think this discourage people even more from clicking on your ads. Secondly, put the adsense content tags on your site between the content which is important to get more targeted ads. If you don't understand this send me a PM.
Smack in the middle of your content, right where a readers eyes naturaly scan. You'd be surprised at how many people never even look at a sidebar.
Here is the real adsnse heatmap address. Wonder if this constitutes copyright infringment. Will Google go after every webmaster who liberally borrows from their pages then displays adsense? Hmmm Good question. https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=17954 And you don't win at Adsense by placing ads all over your site. You win by placing ads in relevant places that will be of value to your visitors. People do not visit sites to click on ads, they visit to find relevant information. When presented properly and if the adsense ads match the content they are seeking you get the clicks you are so desperately trying to get.
In order to find out the best placement, try it out with different combination. There is no general rule that will work for all. It's a trial and error effort.
I always change the placement of ads on my pages and i always found some difference in earnings after changing placement, so its affects alot. just manage it around your template