OK, how about this. I've heard an interview with some
SEO guru yesterday. He answered 5 question related to
SEO. One of them was about keywords and how many keywords should a webmaster aim in order to get better SERPs.
He answered that it depends on the popularity of the site. The more backlinks and traffic you have, the more potential you have. If you don't have potential (traffic+backlinks) you should aim for only a few keywords, study the competition and see how much traffic and backlinks they have, then when you reach them, you could aim for more keywords. Getting good for one ~ three keywords at the beggining will generate traffic under that keywords, while 40 keywords won't generate that much, cose the importance of your site woun't be good enough for google.
Hope this helps...