How Do I Measure Keyword Difficulty?

How do I measure the difficulty of a long tail (3 words or more) keyword? Lets say the Google Keyword Tool has a keyword that gets 5000 searches a month and the "competition" column says "low". Does that refer to PPC competition or SEO competition. I plan to create a small site specifically for one keyword. How can I measure how difficult it will be to get to page one? I know I can look at pagerank, but what if its a PR0 page on a site that is PR5. Am I competing with the PR5 or the PR0?

thanks

The competition mentioned in GAKT is ppc competition and not seo competition to find low competition seo keywords use traffic travis or market samurai and it is better you select keywords that have page rank < 3