Roughly speaking, quality backlinks just another part of your site that google looking for, good content also very important in long term, if peoples love your site it's more easy for them give you link love
In other to get good Google PR, you need backlinks from good website or blog that are related or relevant to you site with good PR. You also need to have a good link building program in place, so that you will be get quality backlink for your site from time to time, in other to improve your PR, when even Google update it.