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"
I wonder if something like this would work instead and make these things basically silent.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835553002 The cfm is 119. So, it's pretty close, particularly if you use two of them.
Edit: The only problem I have had so far is that I have not been able to get OpenWrt to recognize the fan on the second blade and control the RPMs accordingly.