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PageRank is a specific and formally defined number that is calculated from the quantity of inbound links, the PageRank of the linking pages, and how many links are on each of the linking pages. Since the PR of any page involves the PR of the linking pages, this is a recursive calculation that needs to keep "looping" until the number approaches a limit -- and there is an additional factor in the PR definition called the "damping factor" to make sire that the recursive calculation doesn't take off for infitnity.
Nothing else goes into the PageRank (PR) number itself, although how any give page will be ranked on a particular search obviously uses many more factors, including the possibility of those javaboy mentioned.
I quickly made it to #9 for the term.
Since July I have added at least 300 many with good PR and I have noticed no change.
I think something has changed in the importance of backlinks.
Or since the last major update Google has more or less ignored new backlinks.
Personally I think PR is overrated. Taking advice from a toolbar to pick your link partners is similar to using stock picks from 2000. It is way out of date.
The most important factor in backlinks is the anchor text, so if you added 200 links with proper (keyword-rich) anchor text, they help more that 300 links from high PR page but not related anchor text.
But PR still is important, but the ideal is to have both PR and good anchor text. It took me a few weeks to got #1 in phrase with 172000 results with just two PR4 links, both with anchor text containing the phrase.