Ranked in top five is good indeed. However, you need to decide what your goals are. If you get enough clicks - don't pay Overture; if you think you'd be better spending some cash and getting more hits (leads, sales...) go for it.

I run one site that has #1 position for two major search phrases on Yahoo! in a multi billion dollar industry segment. None the less we do pay a few bucks a month to leave no room for competition above our placement. The fact is, even #1 position is two (and sometime more) positions below a sponsored spot. By the way, we never paid Yahoo! or anyone else for our link inclusion. My guess, the relevance and a good ration of key words in a header/URL/META tags/body, still work.

On the other hand another site I manage paid for inclusion of three links to iNeedHits.com to appear on AskJeeves and their network. They just sent a message letting us know of "renewal specials". I checked our stats and apparently we did not get one click from that deal... I just finished writing to them letting them know about that waste of money.