I have a problem with the fans that run in my desktop computer. I was wondering what this could be?

Over the past few years, I have upgraded my graphics/video card to BFG Tech 512MB AGP Card, upgraded my memory from 1GB to 4GB with installed memory coolers, and my main concern is that my desktop is running its fans really fast which makes is sound like a power plant. I was wondering what can I do to correct this problem. Do I need a higher power supply or some fan controller. I mean it does the job I wanted it to but is there any way to silence the sound of the fan? Any advice, suggestions, etc?