If you are planning to overclock DO NOT replace the stock fan unless you are getting another fan with the same CFM

The stock fan pushes 130 cfm
Anything less and you run the risk of overheating or damaging the unit

I added the same exact model as the stock fan, the back blowing air out
They are about $7-8 on eBay

Just search for "4715KL-04W-B56"