Contrary to the "Low air resistance, low fan speed, low noise technique" post, I found that putting a filter in my case have made it slightly quieter, though at the expense of slightly higher temperature (about 2–4°C higher it seems).
My case's "front intake":
The "front intake" with cardboard filter box:
More photos of my computer
I think I'll go to an art supplies store and get some mesh and try to make a proper grille.
EDIT: Sorry for the edits. I tried to use Yahoo for the photos and it was difficult to get it to play nice...