Ah... but when you get the layout just right... three ad units and one link unit can really light up your checkbook.
Amen to that Will. I have a couple sites that work best with this method also. So testing a site for effectiveness is important.
Unfortunately not many do this and it is one of the most important parts of setting up your site. Take a little time people to make sure that your ads are in the most effective spots. This does not happen over night. You have to test and this may take a month sometimes or more. Make sure you are accurate as the results you get can mean the difference between good money and bad money.
Little changes can mean a lot, like putting the little graphic banner into The Internet Search Engines FAQ. That draws visitors eyes to the top ad. Mmmmmmm...