Well, don't know how to get PR5, but here's a thread explaining how to get PR6 In the end it boils down to "how google does it". One could guess "how many links you need" but those are just that: guesswork. Google is the one who defines pageranking, and I'm pretty sure these guys take a LOTS of things into account when setting pageranks. I don't think "buy X PR6 links, Y number of PR5 links and Z number of PR4/3/2/1 links" can really lead you to magic formula that works 100%.
Well, from what I understand, PR is important in securing high SERP, and staying there. My blog managed to get a PR of 4. How I got there? Submit lots of articles (50+) and directories (50+). After 3 months got a surprise one morning to see my PR got to a 4. You may want to try that.