In all my expericence in the PPC arena I would have to say that the top 3 positions are where you will generate the most click throughs so if you just looking for traffic thats where you should be (running an adsencse site or something). However if you are into a business that requires sales the best place for conversions is between 4 and 6th position.
It has to be said that the ad itself must be build around maximum click throughs so be sure to use questions, keywords, and above all the scarcity factor.