As I've mentioned here before, I'm making a network of dozens of sites (and interlinking between them). Currently these are hosted all on the same server. I'm wondering if it's worth it to spread these out over 2-3 different servers (different C-class) to try to get the extra SEO benefit from the links between the sites.
It would help yes, but it just depends on how much work you want to put into the sites. Internal links and links from the same server are still worthwhile, and if you are not planning on running these as major authority sites or working on them for 1000s of hours then I probably wouldn't bother - you can easily enough gbet backlinks from various sources, different IPs, domains etc without going to the trouble of setting up different hosting, and mapping out what should link where etc.