If you are overclocking you want a custom heatsink fan that will cool better than the stock one with additional case fans.
If you arent overclocking then the stock cpu heatsink and fan is fine and its a good idea to have extra case fans for optimum airflow. It will largely depend on where you have your case and if theres good airflow and a decent room temperature.
If the room temp is quite warm and its in a restricted airflow space the case fans are a must...
You don't need a 'custom heatsink fan" to overclock. I recommend seeing what your stock HSF is up too before purchasinig a 3rd party HSF.
I have an Intel E4300 in an EVGA 680i motherboard. The CPU comes stock at 1.8Ghz (9x200); I've overclocked to 3.2ghz (8x400mhz) or 3.0ghz (9x333mhz) both using the Stock HSF.