I found a guy the other day that had built his own network of WP sites (40+) linked to one another. There was duplicate content all over the place but I'd say 80% of his sites ranked on the first page of Google. His linking method was sidewide flat html links on the bottom of each site. The key to his success (I believe) was different IPs for each site. Most were just DNS created off a random domain as opposed to using hostgator, etc. How is he creating so many different DNS?