Here's a great article explaining the 3-4 in general, as well as the different schemes run by different teams.
http://www.milehighreport.com/2008/5/7/481970/mhr-university-modern-3-4 I think we'd all be well served to refer back to this article, as well as the comments coach and Dom have made about fitting the scheme to the players rather than vice-versa when we talk about our guys "fitting the 3-4". (ie. Kampy can't play OLB, Harrell's not big enough for NT, etc.)
You only need a mammoth NT if you plan on running a 2 gap system along the line. Dallas, however, runs a 1 gap and gets by with a 305lb Jay Ratliff. Whether we'd be better off using Pickett as a 2 gap or Harrell as a 1 gap is the type of question Dom will be evaluating over the coming months.
The point is not what we should or shouldn't do, but simply understanding that whatever defensive scheme we ultimately decide on will be largely dictated by our current personnel rather than the other way around.