The rheosmart 6 is a great controller and will function as Tsumi has mentioned. That is what I use. You can build a custom one if you like too...there are a few tutorials out there if you're not sure how to do this. The least expensive method though is using a preconfigured unit as you have.
The rheosmart is controlled from one PWM header. If your GPU has a PWM control from it, you can use that to control the rheosmart. Most GPU's I believe are voltage controlled so you'll use your PWM port from the CPU header. If you use an ASUS board, they have FAN Xpert which works rather welll. You'll have to find via trial and error how you want your graph to look which controls the PWM signal. It's not too hard to set, just run a few stress tests and adjust the graphs as needed.
Most of the time, unless I'm gaming, my fans actually run between 550rpm to 750rpm (I have 8 fans within my setup). It's dead quiet when they are turned down low like this. My CPU temp runs mostly around 20C and the GPU around 27C. I haven't overclocked the computer yet which is why the temps are that low.