For the i920, you want to look for the sSpec Number SLBEJ. This is D0 stepping which pretty much makes it 4.0Ghz capable on air.
http://processorfinder.intel.com/detail ... Spec=SLBEJThere are many guides to decode the intel numbers, but look for a specific week of manufacturing. When the i7 965+ or Xeon quota is filled, they at times slip some of these quality cores for lesser frequency processors which are in greater demand due to price.
Spending a few dollars more can get you the i930, slighter higher validation and for even a few more above that, you can get the Xeon X5520.
If you're going for the monster OC with an air, or even watercooling solution, I'd spend the money for a Xeon.