do your keyword research with
http://tools.seobook.com and see how different keywords get the traffic you want. But the higher the traffic the more competative the term will be. You might get more traffic ranking #1 for a less searched for term than you would raking #30-#50 for Free Porn or whatever. Plus you can always change it and after a month or so it will adjust and you can rank for different keywords as your site grows.