Should I keep the stock cooler on i5 or get a better one. I'm not planning on OC.
More aboutstock cooler
If you are not planning on OC-ing, then stick with the stock cooler, it'll be fine. If you plan on OC-ing in the future, you will still be able to get a minor OC on the stock cooler, but an aftermarket cooling solution is recommended.
Stock Cooler is adequate for maintaining temps for non OC Processor. They are normally quiet too.You don't need to buy an aftermarket cooler.
stock then! Even 3GHz shouldn't be a problem on a stock cooler.