I've worked with many clients, large and small. Some with a big ad spend, some with a small one and soome with no ad spend at all. I never saw any correlation between ad spend and ranking. In fact, I'm sure that if there was any such thing one of the larger agencies would notice it and THAT would kill Google. As soon as the lemmings stopped trying to cash in by buying ads, the agencies would shut down the flow of big advertisers and start talking to lawyers.