How much rewriting should you do? Will just writing a brief introduction to the article and perhaps adding your own comments after the article do the trick?
Google makes it clear in its webmaster guidelines that it doesn't like duplicate content. Although Google isn't doing a brilliant job of filtering out duplicate content at the moment, I think it's inevitable that Google will get better at doing this.
So the more genuine rewriting you do - not just changing a few words here and there - the more success you're likely to have in the search engines.
Because of the fast rate at which people are snapping up article site memberships, I suspect that most article buyers are doing very little rewriting. For us, that's good. Our unique, carefully rewritten articles have much more chance of ranking well.
