There's been a lot of discussion on this since time immemorial. There are lots of ways to reduce system noise.
In your case, the dual CPU HSFs are probably the number one culprit. You should look at the
Thermalright AX-7,
SVC GC-68 or Alpha PAL8045 with a 35-40 CFM fan from Panasonic or YSTech (see
GiZzO's Fan Database). Hopefully there's enough clearance on your board for two 80mm HSFs.
Next, look at the power supply. For a dual processor, I would recommend the Enermax models over the PC Power & Cooling ones because Enermax have a 90mm intake fan and an 80mm exhaust fan to help cool the CPUs. PCP&C only offer a single 80mm exhaust for the power supply itself. Other folks like Antec, but their MTBF is lower than Enermax or PCP&C.
Then try to find out how many CFM the case fans move. If possible, replace them with
PCP&C Silencer fans. Otherwise, look at the various Panasonic Panaflo and YSTech models. If the exhaust fan is moving air through a grate cut out of the case, cut out the grating and install a real fan guard - grating cutouts cause a lot more turbulence than fan guards, and hence more noise.
Finally, install rubber washers and other vibration dampening on vibrating parts. Install washers between fans and the case, between the power supply and the case where the screws attach, and between hard drives and the case. Install padding like Surface Gard adhesive strips between the power supply and the case to prevent side-to-side and top-to-bottom vibration from adding unwanted noise.
If you're still not happy, look into replacing the fan on your GeForce with a lower-noise model, and consider the Zalman NB-32 or NB-32J northbridge heatsink to get rid of the HSF on your northbridge. You could even look into widening the exhaust fan opening to use a 90mm or 120mm fan instead of the standard 80mm. Bigger fans can push more CFM with less noise.
Good luck!