I dont really. I just wanted to upgrade to cool my system better, but I am not really sure there is a point at the moment.
If you've already got the optimal fan setup in your case, replacing the fans won't show a significant improvement.
Front: 2x intake
Rear: Exhaust
Top: Depends on your cooler. With an air cooler, you don't want the first top fan 'in front of it', or it ends up pulling cool air out before it can feed the cooler. In that case, it would be better to remove it.
Bottom: The weakest intake - practically useless. The restrictions are too strong. You'd have to run the fan at max to get any reasonable amount of air through here, and it would still be weaker than the others...
Side: Disregard if you have the window...