The website I've taken over has many marketing approaches that have already been taken. We use Overture, Google Adwords, are a premium sponsor on Yahoo, use bcentral's Submit It, and have a few rec. links. We spend around $300 -$500 a month on all of this. We rank well with just a search in almost any search engine. We are a small company, small website (35 pages.) My question is, when is it overkill?
It isn't all necessary. I have some sites that do very well with PPC but terrible everywhere else and others where PPC wouldn't be worth it but have great search engine rankings. You can have a successful site using any one of those methods but if the others can make you money you should go for it.