It's a toss up in my opinion what
PR is really worth below say a PR8. I've had plenty of
PR 7's that fluctuate in the rankings during every reindex, sometimes dramatically falling out of place. Which isn't that big of a deal until you see a few sites with
PR 5's pulling top spots.
More specifically, there are way too many factors for SERPS than just
PR. My theory is that Google not only heavily weights certain links from certain "trusted sites" but also uses their Google toolbar to gauge where users are visiting, and also gathers plenty of contextual information on the site. Google has a very good idea of what sites are popular and most of the very large market categories have had relatively few changes in the top 10 listings.
Don't get me wrong you can increase
PR to a 7 through a few great link exchanges and have a ton of great content that instantly gets you top rankings...but Googlebombing is not what it once was and the way Google has defined relevancy in its algorithm has consistently adapted to fend off manipulation.
However...since you're just looking for ways to increase
PR, it never hurts to find top
PR sites and offer them money...that and you know, maybe creating a lot of great content that people want to link to. It's not easy and it definitely takes time.