Most cases will ship with at least a few basic fans, which should give you adequate cooling if you're not going to be doing anything too crazy. However, if you'll be pushing your machine for performance, chances are you'll need a decent set of fans to keep your temperatures ideal. There are two main types - Static Pressure and Air Flow. The first type will provide increased pressure to increase airflow through restricted airways, such as a radiator, while the latter will provide more overall air movement in an open area.
Things to look for: