world4health
Selecting keywords is really a two-part process. First, do the research on which keywords people are using.
Then check the SERP's and see the competition. If the competition beatable, go for it; but if the competition is solid, reconsider.
Like, for "jobs":
www.jobs.com
www.monster.com
www.hotjobs.com
www.usajobs.opm.gov
www.careerbuilder.com
Those are all PR8 or PR9. You could beat those, but the resources required to do it may not be the best way to go. If you have the resources to beat them for that one search term, you could use it to beat them on a number of less competitive keywords which would drive more traffic overall.