The stock cooler tends to get loud on the FX-8120/8150 your best bet would be to get a good aftermarket cooler. For mine I choose to get a XSPC 750 RS240 water cooling kit. Even with four 120mm fans in a push/pull it is still quieter than the stock cooler. Running at 4ghz with a 20x multiplier my temps are at 29c idle and have not gone over 35c full load.
A friend of mine build the same system but got a COOLER MASTER Hyper 212 Plus http://www.newegg.com/Product/Product.aspx?Item=N82E16835103065
and has had really good results with it and at $29.99 it is a good price and is still quieter than the stock cooler.