I'd suggest following
Matt Cutts' blog. (Matt Cutts is the head of Google's WebSpam team.) In
this entry in particular, he explains how and why a newspaper that had a Page Rank of 7 was demoted to a PR3 because they failed to use
rel="nofollow" on paid links.
Keep in mind that we're talking about a site that had the kind of content and web trust that would generate a PR7 if it weren't for the links — if your site is nothing much more than a link page and the links are not marked nofollow, you're not going to get anywhere near PR3 no matter what you do. You'd be lucky to get a "real" 1 (it's possible to blip up to a 2 or even a 3 temporarily, but it's not going to last more than a couple of days at most, and any inbound links passing PR value to you will be demoted as well). Since the whole aim of your site/page seems to be passing Page Rank to lift the search value of other sites, it's pretty much exactly what Google is trying its best to squash. The whole business model of Google search relies on high-quality search results — that's what keeps users turning to Google, and users are what makes AdWords valuable to advertisers. If they let the result quality fall, they'll lose users, and that means they'll lose advertisers.