The sexiest part of a 3-4 defense is its outside linebackers. They're the ones that gets the sacks (and probably, the cheerleaders).
IMO, however, the most important part of a 3-4 is the defensive line in general, and the defensive tackle in particular. When I think of a 3-4 DT, I think of a space-eater: someone who can take on two blockers and not get pushed backwards. Vince Wilfork, Grady Jackson, Casey Hampton, etc.
Do we have that guy? Pickett is the obvious choice, but what about Colin Cole? Justin Harrell? Johnny Jolly? Physically, we have several players who could fit the bill:
Pickett 6-2 330
Cole 6-1 330
Jolly 6-3 320
Harrell 6-4 320
Bledsoe (PS) 6-3 329
Is is possible that one or more of these, while not excelling in a 4-3, could be a good fit for the 3-4?
And if not, who's available that we could pick up?
Obviously, Albert Haynesworth. But what about Tommie Harris? Rocky Bernard? Tank Johnson? (OK, forget Tank).