About.com asks the question – should those wanting to optimize their site target single keywords or keyword phrases?
“Consider the keyword “cars.” A quick search for this phrase at Google reveals nearly 20 million web sites competing for a ranking for this phrase. Since studies show that few consumers make it past the first few pages of search results, that means that all but about 30 of these web sites are unlikely to see any traffic from this phrase.
Apart from the fact that even the most skilled SEOs would be hard pressed to earn top rankings on this keyword, it’s also not a very targeted phrase to optimize for. Will the searcher who types “cars” be looking for pictures of cars? Car sales? Car events? New Cars? Old Cars? The list could go on and on.
Web surfers tend to learn very quickly that in order to find what they are looking for, they need to target their search queries more carefully. So…get specific with your keywords.”
Read more at SEO 101 – Beginner’s Guide to Keyword Research