Even as a dude with an undergrad degree in math it took me a while to realise that PageRank is on a log scale rather than a linear scale. So a PR 6 is worth hundreds more than a PR 3 link (assuming niche relevance of the sites etc.) SO what this means is, while owning a few PR 3s is very effective, it can definitely be worth saving your money to buy a few PR 4s and 5s. I personally stay away from buying domains that claim to be PR 7 or above, 5 is the maximum that I will look at.
There are several good references out there and I use the following PR chart every day - http://mlmbigbrother.com/wp-content/uploads/2012/03/foxhound-seo-pagerank-link-chart.gif
Pr is very important, but its much easier to rank a website with good On Page SEO these day. By this i mean good internal links and a great silo structure. It takes a lot less backlinks to rank your site if you get this right.
there are so many things to take into consideration before buying an old domaine with high PR , if it is sandboxed do not even think about buying it
Just remember not to place your eggs in the same place. Internet marketing has it own mysterious risk.