Although PR from other sites do up your page rank, it's not quite that simple. Google uses a quite advanced algorithm. You need more than 5 links of a certain PR to get that PR. For example, If I start a site today (PR 0) and I purchase 5 - PR 6 links. My PR will NOT be 6 at the next update. Focus on a comprehensive SEO package rather than relying on high PR links.
Get, lot and lots and lots of backlinks...to the tune of several thousand.... Take my sig as an example...
Create more pages for your websites..optimize your tags... keep internal link structure perfect. start bookmarking, directory submission.. start article posting... your pr will surely increased.