My advice : Ignore PR. It can lead you to chase something which in fact is not important.
Linking building, that is important. Free ways :
Search Engine & Directory submissions (1000’s out there)
Article Directories (100’s out there)
Get involved in forums, blogs and have it as part of your signature.
Link Trades
Social Networking
Start Pimping your site out.
I should also add, that when looking for link backs, I consider age of website, how much traffic it might be getting, if it is an active website and gets updated often.