B2/b3 refers to an internal revision system, of sorts. A chip will start on say a B0 state. As little changes are made to the chip, over time, the numeral value changes, B1-B2 etc. If a larger change is made the letter will change, C0-D0. These changes can be both helpful or negative to the values of the certain chip, OCing, heat, etc
I agree with the rest, and add if you are considering buying a faster dual-core, instead of overclocking your current, you may not be of the sort to realize a BE's potential. I'd personal keep your current unless you readily need to upgrade, for performance reasons.