Nzxt phantom full tower fans

so as u know i have the nzxt phantom and have some questions about the case fans.

my pc specs:

Nzxt phantom full tower

asus z77 maximus v formual

i7 2600k

16gb ram

Evga gtx 670 ftw


1) I wanted to know the actual speed of the 120mm fans? 

Now my temp for gpu while i was gaming was 70 celcius max and i want to know if thats good or not. I would also like ur opinion on my decision on changing the case fans to better ones. If yes can u tell me of some good fans 120mm and 1 140mm for the front. And if u think i should not change my fans can u explain y?


Thank you =D