Not that simple - if a single link was PR7 and a bit higher and no other links going elsewhere then you would receive PR7. But PageRank itself isn't much good without being relevant to your topics, and there are 100 other ranking considerations to factor in.
If it's the former, why not get a PR7 directory link from ODP, Yahoo, etc. and be done with it?
For one thing, you can't always get a PR7 link from either directory -- there are plenty of sites with a link from both ODP and Yahoo that themselves have a PR4 or less for their own index page.
But more importantly, the "ultimate goal" isn't either of your options: it's driving targeted traffic to your site. One means to that end is achieving good rankings for specific search terms. To do that you need links from high-PageRank sites to build your own PR (since we're talking about Google), but you also need links from related sites. And don't forget that another means to your end is direct traffic from those links.
PageRank tunnel vision is too easy to fall into -- if the goal is link development, work to build a strong set of links from good sites. PageRank will "happen" naturally.
And you can't "be done with it," because your competitors won't be. Next month they'll presumably have more backwards links; you'll need more too if you expect to maintain or improve your positioning.