Adsense and similar programs need a lot of traffic. Therefore, in many cases, you might want to target high volume single keyphrases. On the other hand, if your targeting low volume keyphrases then you want pick phrases which earn a bigger commission. For example, an attorney or insurance site might be worth 40 dollars a click.
As for me, I'll stick with the high volume low paying keyphrases. If you need to find out about keyphrases then check out the Google traffic estimator.