I have heard that the higher the number after the "C" the higher it can overclock, so I want a C3 product.
I think Psycho dropped that little nugget of wisdom on you without any explanation, and while the statement is factually correct it ain't anywhere close to the whole story.
PhIIs come in 3 flavors: 1) C2 stepping; 2) C3 stepping; and 3) EO stepping (the Thubans).
The C3s have improved voltage characteristics. C2s and C3s generally over-clock the same but the C3 will get you there with less voltage. My understanding is that EO introduced lower-capacitance gating (requiring even less voltage) so EOs tend to go as high (or higher) than C3s with similar/lower voltages (and 2 more cores!).
You may determine the week of the year, day of the week and (most importantly) the location on the wafer where the chip comes from. Chips from the center of the wafer are considered more 'cherry' and of higher quality.