Well, it depends on whether you are talking about one domain or numerous domains to manipulate search rankings. I am hoping you are talking about the former (one domain) as this is seen as a legitimate strategy for a website. However, in choosing your domain you really need to take into perspective the following:
If after answering all of these questions, it is fundamental that you select a keyword rich domain, then this can add greater power to your website offering.
If the question was directed at purchasing multiple domain names for SEO purposes, my recommendation would be to transport you back from 2002 to the present day as these techniques are not working as much as they used to. Google have publically stated that they are working hard to reducing the power obtained by keyword-rich domains.