I bought a Celeron 633 and it was CCO (1.70v), but would not overclock. I have been able to push my PIII 700 WAY past 1GHz, and that one is cBO (1.65v). So even though people say the newer cCO overclocks better, I seriously doubt this is true. In fact, I once speculated that the cCO was actaully a relabled cBO that would not pass at 1.65v and was bumped up to 1.70v. If you order the standard CPU it will likely be cCO because most of the newer ones are. If it turns out to be cBO, don't worry, there is not much if any difference in overclockability.
I would not pay 5 cents extra for cCO.
Cast not thine pearls before the swine