Page rank is one of over 100 factors used to determine how sites rank in the SERP's, read up on it, this is an old topic going back to 2003 kids.
Start reading here
Page Rank fanaticism and focusing on it actually can hurt webmasters when they chase it, make your sites for the users and quit your worries about
PR and you will be much better off in the end, buying high
PR links and trading links to garner
PR will actually harm you and is against G's guidelines.
Matt Cutts on the 100 factors Google uses in ranking websites for the SERP's