Sites don't have page rank, pages have page rank
But then again, I'm sure you already knew that, just pointing out the obvious.
Indeed, a contextual link (inside the content) from a PR 6+ PAGE is super valuable.
Site wide links are actually less valuable these days because many blogs have
sidebars and such, and it's not hard to get into the sidebar for a sitewide links
due to things like the "blogroll" and "recent commenters" widget.
So in-content contextual links are THE SHIT right now.
They are the best type of link you're site can get. But there's no reason you can't rank without them... the second best type of link in my experience is probably deep links from High PR authority sites like profile links.
So basically, it's a matter of figuring out how competitive your niche is for SEO and whether it's worth it based on the return to pay for links from high PR pages... if not, then just start doing other types of SEO like profile links,blog commenting, using blog networks and press releases.