Things you can do to quiet your computer:
1. Change your PSU to a quieter unit that vents well, like a Seasonic Super Tornado or S12.
2. Get the biggest honking heatsink that will fit on your CPU. The larger the CPU heatsink, the slower the fan you can put on it. Slower fans = quieter. Something like a Thermalright XP90 or 120, coupled with a Panaflo L1A (or similar) fans.
3. Put your Northbridge heatsink fan on a fan controller. Stock NB fans usually spin at ridiculous speeds (6000+ RPMs) and are noisy as all heck. They really don't need to spin faster than 3000RPMs.
4. Get a quiet fansink for your vid card. People around here like the Zalman VF700, but I like something that can duct heat from the GPU straight out of the case, which will make the rest of your computer easier to cool, like an Arctic Silencer.
5. Change the rest of your fans to quieter slow-speed models like the L1A.
6. Use rubber washers to mount the fans to prevent them from transferring vibration to your case, so you won't hear a resonant low-frequency buzzzzzzzz.
HTH.