I don't have an answer to that and I suspect that just when someone figures it out, then Google will change their algorithms of measurement... again.
The highest I have personally managed to get any of my websites is PR3, but to keep it there, you really have to be adding new, unique content regularly and continually building new back links. You can't simply build up your links to a certain point, then think you are done. Some links fade away and I wonder if the search engines start giving older links a lower value. Of course, the better quality a link is, the better it is for you.
Some marketers do a search for their top key words and when they identify other sites that rank high for those keywords and also have a high PR, they try to establish links with those sites.
GT








