Keywords and Description meta tags were the first factors that affect your ranking in search engine results, but now they are useless.

Of course adding meta tags will not harm your ranking but will not increase it too.
Google and MSN do not look for meta tags, but they play a very small part in Yahoo ranking.
Google sometimes shows meta description tag in search engine results page.
So our advice is to use them as an organization tool to help you organize your web site on page basis, but if you tried to use them to increase your ranking you will get no results.