Im planning to build a new comp, will i need to buy a separate heat sink?

As long as you are not going to overclock and your case is adequately ventilated the stock heat sink will usually do the job ( though make sure you use good thermal paste.) On the other hand, even an aftermarket heat sink in the 20-30 dollar range is usually far better than stock. So, you don't need it, but for such a small cost it is good insurance if you drive your computer hard.