For me, to achieve a silent while not too bad on cooling case on the crappy centurion 1 airflow design, this is what I did.

Fans.

- 120MM intake, 120MM exhaust fans. Panaflo at 7V. I oiled all the bearings of my fans.
- 80MM chimmney (exhaust fan lar) on the top at 5V
- 120MM fan, sucking air from the bottom of the case. I cut a hole at the bottom. 7V.
- Removed all grilles, except for the back and top exhaust. Nobody stupid enough to put their fingers through, but precaution anyway.

Sponges.

- One relatively thick layer behind motherboard tray.
- One layer on the right side of my case, excluding the window.

Grommets.

- Grommets for all the fans, to reduce the vibration. I ran outta gromets for my top exhaust, so I used those rubber type double-sided tape and just screwe through.

So far, so nice. Doesn't make more than a hum thanks to the 7 and 5V fan mods.