Pagerank has absolutely nothing to do with the traffic a site gets. The ONLY thing that determines Pagerank is the domain's link profile. If you have a good number of high pagerank sites linking to your site, you'll get a good pagerank.
I have a PR5 site that gets only around 20 visitors a day, and a PR1 site that get 1,500.
As for Alexa, it's bullsh!t. Alexa measures the number of people that have the Alexa toolbar installed in their browser that browse the site compared to other sites. Because of this it is VERY easy to manipulate. I can take a brand new website with 0 visitors and give it a sub 100K Alexa rank within a few weeks.