Wait to see what your temps are like before you buy more fans, although those will go well with the included ones. They are slow fans, which means they don't move a lot of air, but they are extremely quiet. 38.4 CFM is definitely low for a 120mm fan, but may be enough. Unless you plan a high OC, you may not need them, and you'd probably be better served by simply replacing the included front and rear (nvm the top) fans with faster ones.
I'm running an insignificantly-OCed 970BE right now, in a case with similar slow, quiet, front, rear, and top fans, and my CPU idles at 30C; Prime95 takes it up to 48C and I still don't hear any of the fans.