I depends to a large extent upon the niche and the value per action.

However, as a general rule, I'd be looking at keywords with more than 500 monthly searches and less than 50,000 competing websites.

Of course, if the sites on page one of the search engines are well established portal sites with masses of links, then it's probably not worth following up.

John

Do you have any metrics for deciding what is too broad or too narrow?