Depends on many things. Your link structure etc...
If you have multiple sites and want the SE's to treat them as totally diffierent pages interlinked...you'll need sepperate IP's for starters...and perhaps different Class C's
If you're at the point where you dont about this though it probably wont do you much good to have multiple ip's/class c's
I would first do some reading. And then start out with a dedicated server with 10 ip's or something.
But without knowing exactly what you're attempting to do its tough to say