A lot of people have the misconception that Google PR actually affects where you show up in the search results. In fact Google PR does not affect where Google places you under your keyword or in their index.
What PR does is tell Googlebot how often to visit your site and crawl your important content pages. The higher PR you have, the more often you get crawled by Googlebot.
In order to increase PR you need backlinks and page popularity. The more people link to your pages, bookmark them etc the higher PR you will usually get.
More BL's does help your search engine ranking, so it serves two purposes by building links. You potentially may get a higher PR bringing Google to your site more often, and you may get higher placements in SERP's under targeted keywords.
Here is a great thread explaining more on PR and has opinions as well:
Google's PageRank (PR) Explained
Build back links and you will see more green and better ranking, but the ranking doesnt come from the green!!