Probably not. I added another 120 in the front and another 140 in the roof and temps didn't really change that much, 2-3C on the GPU at most. But it will depend on what GPU you have, if it has a a blower cooler only the intakes in the case matter really. I wouldn't put a fan on the side or bottom of the case though as they make it vibrate like crazy.
If you want more fans look at the Arctic Cooling F12. Super cheap and look just like the 120mm NZXT fans once you take the stupid label off.