I use PPC extensively, and have found Overture to be one of the WORST investments. If you have a keyword that you want to use, you can be sure 100 people before you have also thought of the same keyword...resulting in the cost for a good rank to be way to high.
Instead, I use a number of the smaller engines. They give me good traffic...it converts..and it is still priced reasonably.
Here's a link I use with all the major PPC players, and a number of New Account Bonuses.
PPC Search Engines