Nowadays many guys are looking for forum posters in freelance websites. Does posting numerous posts related to SEO, Internet marketing techniques etc, increase ur PR in google and thus bring more traffic to your website from ur signature ?
You will receive quality backlinks which may even get you some clients if you just keep backlinks in signature links in such forums. But if you are asking for developing a good Google Page Rank, I would suggest you to look here: http://www.kenkai.com/google-pagerank-table.htm
Great link.. blogged it. Sig links are fantastic for getting the right anchor text, PR, and traffic flowing into your site. Get some catchy header alongside your link and your good.