On the cpu it will not only say a speed (like 1000 MHz) it will have a stepping right after that, such as AXIA. AXIA and AYHJA are particular steppings which OC very well, most steppings will not do as good.
The line after the speed and stepping is a series of numbers and letters which indicate date the chip was made and where it was made. A letter Y is considered the best (generally) at overclocking. Also, any letter will be better than a number.
I have a 1 GHz AXIA Y, which is a good overclocker. SO far I have it at 1.43 GHz, but with my new HSF I hope to get 1.5GHz or more. I bought my cpu from pcnut for only like 160, which is a STEAL even for 1.4 GHz.