is Forum Signature help on SE Ranking?

I have myspace sites and one of sites used to ranking 10th on Google with major keyword “Myspace Layouts” last August, 2006.  Since, then I am trying to increase backlinks with anchor text “Myspace Layouts” and site even got listed on Yahoo Directory.  However, site drop to page2, then page3, last month site listed between page 4 & page 5.  Today, it is gone to page 7.  I am very fraustrated with this site.  I thought I was doing ok with SEO for this site, and it is not working even with PR8 link(I bought PR8 link for the site).  I am looking into how to improve site rank.  My first try is buying some forum signature, do some link exchange.  I search little bit and many users said Forum signature will help..will see about that.