Hostgator saying that the mail runs on a single IP is correct for almost all shared hosting providers. You CAN set dedicated IP accounts to use their own dedicated IP number for sending mail, but MOST HOSTS do not do this. They use the main IP on the server.
And with regards to Pagerank 0 or 3 and what is the difference - it can mean quite a bit! You need to read this article:
http://www.smashingmagazine.com/2007...know-about-it/
Here's a capture of one of the more important lines:
"Google combines PageRank with sophisticated text-matching techniques to find pages that are both important and relevant to user’s search. Google examines all aspects of the page’s content (and the content of the pages linking to it) to determine if it’s a good match for user’s queries.”
So relevancy is the key - it can affect the display of content in the google search!