As CC started to get into, you're looking for three things in a 3-4 DE. Aggressive, one-gap scheme, some pass rush skill and a nice supporting cast. The Wade Phillips (and disciples) school of 3-4 scheme -- think Bruce Smith, Phil Hansen, Michael McCrary (briefly), Marques Douglas, Luis Castillo -- produces the most likely to succeed 3-4 ends. You'll get a Ty Warren or Orpheus Roye or Aaron Smith now and then out of the two gap school, but they often don't have the big sack upside the others do. But, even if you find a 3-4 DE in an aggressive scheme, it makes no difference if they aren't talented enough to take advantage. Finally, it certainly helps if the lineman lines up to the same side as a ridiculously talented rush OLB and/or stud NT to deflect/attract blocking schemes.
For 2007, the short list of 3-4 ends with the most upside include:
Phillips etal guys (sack upside): Luis Castillo, Chris Canty, Marques Douglas, Bryant Young, Ray McDonald
My love for Castillo is well known and I've been pimping Canty (RDE in front of Ware) and McDonald most of the offseason. Douglas is probably more of a tackle upside guy, but Young still has enough pass rush skill to surprise in the 3-4. Olshansky has no pass rush capability and little upside as a result. Spears might succeed in this system, but will have to deal with TE and doesn't have the benefit of Ware.
Primarily 2 gap guys (stud tacklers): Robaire Smith, Richard Seymour, Ty Warren, Shaun Ellis, Aaron Smith, Darnell Dockett
These guys are all interesting, but (except in the case of Seymour and Ellis) don't have the pass rush skill to be anything more than nice tackle options. You may recognize Roye, Smith and Warren as nice producers in the past, but look at their career stats. Look like seismographs -- little year-to-year consistency. You take Warren at cost, I'll take Dockett on sale. I may kill you in the boxscore by year's end. The two gap school is tough to graduate from with any reliable IDP value.