Unless you are overclocking. You shouldn't even be asking this question. Ok. well I can think of one reason... quiet. Most of the time the stock hs/fan is pretty quiet. Unless you get it and find it's still too noisey for you... then you're just overly sensative. j/k, there are quieter ones.

Stock OEM fan/heatsink will be more than enough for cooling. Definatly do go with the retail box, having a warrenty is really nice.