PR 1-3 sites won't do much at all in terms of helping your PR as they cannot increase your PR above theirs, a link from a PR2 site cannot make your site a PR4 per say but will count as a plus point in terms of helping your rankings as it's still a link which is effectively 'a vote towards your site' as far as engines are concerned.
Depending upon site navigation techniques and the amount of pages in a site that are actually indexed you could easily see a PR5 for your home page but whether it spreads through the rest of your site effectively is a mute point.
Navigation techniques employed through a site to connect a home page to other pages within a site will 'transfer' PR to internal pages and in doing so those pages will benefit from higher serps as long as the navigation allows this to happen. Sites using navigation code that cannot be traversed by robots will not see the spread of PR throughout their pages.
If you are only seo'ing the home page then this is not a problem but if you are expecting visitors to enter from most / all pages then friendly and effective navigation needs to be in place.
I hope this helps.
Ray