The stock heatsink usuall will work just fine, unless you are trying to do some serious overclocking.
You do need a case fan. Your PSU will be exhausting hot air from the case, but it is very beneficial to have at least 1 fan at the front bottom of the case helping to draw cool air in as well. Even better, 1 intake fan at the bottom front, and an additional fan at the upper rear to help exhaust the hot air.
You need good airflow through the case to keep the hot air moving out, or you may indeed have some high temp problems.
Fans don't cost much, and they are easy to install.