there has been much recent WebmasterWorld discussion about the keywords meta element. i can't find the most recent thread i was reading on this subject, probably in the Google Search forum, but perhaps this thread has something useful for you: [webmasterworld.com...]
From an SEO point of view I can tell you it's definitely not 'the more the better'. If you want your site to remain visible in the Google search results, keyword stuffing (entering huge strings of meta keywords) is a very bad thing to do. Aim for around 15 - 20 relevant keywords to your site, and for added security try to incorporate some of these in the body text of the page. Google doesn't look too closely at META keywords, but it will use it on some level to determine what your site is about in order to give you pagerank, so they need to be relevant to your topic.