It isn't the 3640, the Droid X has the OMAP3630 in it. Both chips have the same ARM core in them, but the 3630 handles memory better.
It isn't overclocked either. I saw that someone posted about the manufacturing process and bining parts (he was talking about PC chips). It is the same process for cell phone chips. Basically you test the die on the wafer and use the ones that will run at 1GHz (with margin).