New fans

Im using the stock fans for my fractal design refine R3+ one crappy fan from my old computer, The dust filter is barely dirty after a week or two while the dust filters for the stock fans are full of dust.. So I doubt it helps much but it's atleast something  

So I want some new fans..

Fan slots on the case:

2 front 120mm
1 back 120mm
1 side 120/140mm
2 top 120/140mm
1 bottom 120/140mm


I wanna change the 2 in the front and the 1 in the back atleast, Should I put one in the top and one in the bottom aswell? Or is the front/back enough cooling?

Noise isn't a huge problem since im playing music loud,or gaming with my headset most of the time

Also should I change the CPU cooler? I have the stock one for the i5 2500k 3,3ghz

My room is really hot during the summers,But i'll live.. I just don't want the computer to melt down