As discussed in this article, it is a good idea to build each of your pages around its own primary keyphrase, and somehow incorporate that keyphrase into your <> tag. The question is, of course, what’s the best way to integrate it? The problem is that each search engine has its own unique answer.
MSN (aka Live.com) is generally thought to reward very high keyword density, and often grants top-five rankings to pages with 100% density in the title (that is, pages where the primary keyphrase is the only thing in the title bar).
Google, on the other hand, seems to make a point of devaluing pages on keywords that exactly match their <> tag. This measure was most likely introduced as a way to fight search engine spammers who over-optimize for a single phrase, by excessively placing it in their content, headings, and title.
Overall, you need to make an informed decision about which optimization route you want to take for each of your sites. As mentioned in the algorithm summaries, MSN is a good choice for driving short-term traffic and revenue, while Google has a lot more potential for long-term sustainable content websites.
If you want to optimize for Google, my advice would be to go for title keyword density of around 50%, and no greater than 75%. For example, if your primary keyphrase is three words long, you many wish to add another three-word phrase to your title, consisting of secondary keywords.
