I just happen to be doing some research on this subject at the moment: First of all C-Class refers to part of an IP Address: An IP address is the location of a computer/server on the Internet. It is 4 sets of digits, known as octets. The third octet is called the C-Class IP address, for example in the IP 216.230.215.91 the C-Class IP address is 215. More importantly - does it help with SEO? Back in 2001 before LocalRank it became apparent that links between sites on the same server - or even in the same C-Class IP range would be discounted in value. However while some think there is still value in hosting various sites you own among different c-classes Google has said that the sites IPs no longer are factored in and the more I read the more I am finding that C-Class IP SEO strategy is outdated and may now be merely a popular SEO myth: "Actually, Google handles virtually hosted domains and their links just the same as domains on unique IP addresses. If your ISP does virtual hosting correctly, you'll never see a difference between the two cases. We do see a small percentage of ISPs every month that misconfigure their virtual hosting, which might account for this persistent misperception -- thanks for giving me the chance to dispel a myth!" -Google Director of Technology Craig Silverstein (Slashdot interview) So unless you are on the Spammer side of things and want protection from getting more than one site banned at a time - you may not need to invest in Class-C links. But like I said others will disagree - so do your own research, but for now I am choosing to believe Google.
Here Matt Cutt's affirms that this is still true: http://www.mattcutts.com/blog/myth-busting-virtual-hosts-vs-dedicated-ip-addresses/