I want to know when google could ban my site - put it in sandbox or whatever that could hurt me. The question is referring to building links and quantity of links. So if I buy 3 PR6 links for my PR3 website and put them up in the same day.. would this hurt me more than it would do me good ?
You can have a great content article that hits the front page of Digg or something, and that may end up generating a ton of links naturally. Are those sites ever penalized? I've never seen it. I wouldn't say there's ever a "too much" threshold, but I would say that you should buy those relevant links -- I suppose the ones that send you traffic like AfterHim said.
I'm doing it for better serps ... I was first on MSN and now Its dropped on eight place :S and I'm nowhere in google for competitive keywords :S