1. Domain names are not counted toward SEO. Links and quality content are what drives SEO. That is what you should really focus on.
2. Having more than one domain name doesn't help SEO--especially if they are all on the same group of servers. Google knows when one person owns all the same domain names and when they are all on the same servers. If you wanted to try and fool google you might be able to have 500 servers in 500 different locations running 500 different software profiles, etc. However, the amount of time to do that is greater than just making good content.
3. Putting keyword in a domain just makes it easier to remember. Which is worthwhile...