Which fan is making the most noise? It's probably the CPU fan, so . . . buy a fan adapter and have a low noise 92mm fan suck air from your cpu. You'll get as much cooling as a howling 60mm or a noisy 80. If your power supply is noisy, see if you can trade out that 80mm for a quieter (slower) one. To compensate for the slower PS fan, add a low noise 80mm case fan to suck air out of the back of the case. To make sure you have enough air coming in, make sure that you have another 80mm case fan blowing air in from the front. Don't forget about your hard disks, which would like a fan of their own. And a slot fan, like the Cyber Cooler Central System Cooler, is great for cooling your video card and bringing in some more cool air. You'll hardly notice any noise coming out of your PC after you make these simple changes.
By the way, did I mention I run a fan company?
PS: My first suggestion was the best (re: the 92mm fan adapter).