Having a link on a high PR site though isn't going to guarantee you a better SE ranking. Being linked to from sites that are relevant and are already well placed in Search Engines (as a lot of high PR sites rank poorly) is surely more efficient?

If you have a website about cars and buy a link on a PR7 site about the universe (or even from a link farm type site) then your sites PR will improve but your SE ranking probably wont.

At least thats the way I've always been told it works, from what I've read. I'm no expert though if I'm wrong just let me know, in fact I probably shouldn't be saying anything when I don't know if its purt fact or not.