Getting 2.5 gig out of a 2.0a is not bad. I sure would not get rid of that CPU in hopes for another one that "might" do better. You make no mention of what components you're using or what cooling you have or what temps you're running... all of which might affect your overclock.
Anyway, the C1 stepping Northwoods will have a default voltage of 1.525v compared to the 1.5v of the BO stepping. You can also look up the s-spec codes, but the vcore is the easiest way to tell.