As far as linking goes, your links won't be given as much weight if you are interlinking the sites on the same IP, as Google will be able to spot that you own them.
However, having sites on the same IP will help in the SERPS, as if you have an established site on one IP and set up a new site on the same, the new site will avoid the sandbox.
Before, I set up a new site on the same IP as thepokerbank, and the site was ranking well within the first few weeks. However, with another site I set up on a different host and thus different IP, after the site got indexed it barely even ranked for the exact words in the domain name for the first few weeks, and the niche isn't even that competitive.
So if you're looking to set up a new site and not use it to feed weighted backlinks to your main site, then I would recommend using the same host and on the same IP if possible, as it will allow you to get over the intial bump of being sandboxed for a few weeks or months.
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