They will help if your case is aluminum, or a light-weight case.
You might want to swap your fans (example: From 80mm to 120mm) if your case permits. One 120mm fan moves just about as much air as two similar 80mm fans, and the 120's usually spin slower. This reduces noise.
If thermal management is enabled in your BIOS, you can set the fans to spin slowly when idle or doing light work, then ramp up as needed.
You might also look into checking airflow if you have multiple fans installed. You might be able to remove one or two fans and still maintain decent temperatures.
There are also quiet power supplies available from Vantec, Antec, & PC Power & Cooling, as well as low-noise fans. Check their websites, and see the ratings used. Some might even have testimonials.