Depends on your case, cpu, and ambient temperature. Basically, all the fans your case has spots for. The most important part is the cpu heatsink and fan, you want one thats big and copper, with the largest fan you can, and properly applied thermal grease. If you really want to go far, you'll need liquid cooling, peltier, or a vapor cooling solution. Putting the computer in the coolest area possible, like the basement, will help if you are using air cooling, since aircooling can't get below the ambient temperature. Be careful with overclocking, and only do it for fun. Else just get a faster processor.