Having just finished making my case as quiet as possible, I have some suggestions to share with you.
Before you even start, it's probably best if you get quieter fans. There is no real need to purchase "high output" fans that are extremely noisy. Unless your case is the size of the Empire State Building, you shouldn't need 170 cfm blowing in the front. Instead go with a "Quiet" 120 mm fan. Despite their name, "Quiet" 120 mms, are still audible, although not really noisy. For exhaust get standard or, if possible, "Quiet" 92 mm fans. AVOID HIGH OUTPUT FANS LIKE THE PLAGUE, unless you REALLY don't care about the noise (and since you are asking I assume that you do).
First of all, as listed above, cut out the holes/gratings in front of the fans. For all of your exhaust fans, use nothing but a metal grill to cover them. Although the metal grill might cut down on some air flow, the effect is minimal (unlike the pressed metal grates of the case which seem to be comprised of more metal than air holes) and they do not generate excess noise.
For your intake fan, you probably want to use a mesh filter to stop dust and other stuff from being sucked into your case. The thing you need to realize about mesh filters is this: When placed on the INTAKE side of a fan, they do not generate much noise or cut down significantly on airflow. On the other hand, if you put a mesh filter on the Exhaust side of a fan it SEVERELY limits airflow and is LOUD AS HELL! I believe this has to do with the way fans work. The intake side 'passively' draws a lot of slow moving air in from all over, meanwhile the exhaust side blasts it straight out with a lot of speed and turbulence.
As for mounting the fans, you can follow the grommet approach listed above. If you can't find grommets, then you can try using rubber electrical tape and wrapping the screws in that. Probably you can even use cloth or duct tape if necessary.
Another thing I tried was cutting a rubber buffer out of a rubber doormat that I purchased at Home Depot for about $5 and placing it between the fan and the case. The rubber is intended to absorb any shocks or vibrations from the fan, so they do not vibrate the case making noise. I'm not sure how much difference it actually makes, but I'm sure it's worth a shot.
Finally, I placed rubber doormats, cut to size, against all possible areas inside my case. This dampens the internal noise ALOT. My computer is much quieter now with the case on than with it off. Unfortunately, I could not place any rubber matting against the front of my case, or I chose not to, due to the extreme difficulty of cutting it to the exact shape so that it did not interfere with any of the drive holes or the attachment holes of the front of my case. Originally I was planning on placing even more rubber mats on the outside of my case, but I was already so happy with the results of just mounting it internally that I decided not to bother.