The S&P 500 is a powerful representation of today's market, including the 500 companies that fuel the economy. In order to make it into this benchmark, most of a company's shares must be available for trading, market cap should be $18 billion or more, and the company must be profitable. So, to be invited into the index is a pretty big deal. Inclusion also could result in gains for a particular stock over time as funds that track the index add it -- and as more cautious investors see this milestone as a sign ...