PageRank is on a logarithmic scale- Therefore PR4 is 10 times stronger than PR3, and PR5 is 100 times stronger than PR3.
Thus having a billion PR0 links isn't worth as much as a single PR6 link to your site.
Hmm, dont look at the PR - look at how many backlinks the site has and how many of them are relevant. Try to get relevant backlinks (incontent links would be great).
The idea doesnot have to be big. It just has to be yours
As Farrhad said go in only for high PR links, they will help you much more.
Dude, since you are starting up first go for
1. Regular content. 1 post a day. Get indexed in google.
2. Get some basic visibility (for SE's) by social bookmarking.
3. and then get on with link building with good content. Higher PR is good. But initially some good PR2/3 links will be just fine. Target basic keywords.