I do own two computers, one quiet but expensive Phenom-based DAW in a P182. The case is great and pretty quiet out of stock and with a Scythe Ninja 2, 2 Nexus Real Silent fans and a fan controller I'm actually satisfied.
My other computer, an socket 939 Athlon 64 with a nforce 4 ultra chipset is a whole different story. The case is a big Chieftec mid-tower with pretty good airflow due to lots of ventilationholes in both sidepanels and a maximum of 5 fans.
Unfortunately the HDD mounting was bad and vibrations a major issue. It also leaked lots of fan-noise despite undervolted semi-quiet fans.
I tried solving the vibration problem with a mounting technique similar to Mike's. It helped with vibration but added to much heat for the drives.
Cable management possibilities was also a problem so I decided to buy a dremel and mod the case until it was adequete for my needs.
Front Intake - I used a saw to enlarge it, filter is made of cut nylon stocking.
Sideplate - Ventilationholes are blocked of with dampening material.
Interior 1 - Motherboard fan was replaced by a modified P200 CPU-cooler attached with zalman thermal glue.
Interior 2 - Three holes have been cut in the motherboard tray, one above the CPU, one in height with main power connector and one close to the SATA connectors. The HDD cage has been removed and the drives suspended. One fangrill has been removed.
The mess behind the motherboard tray:
Athlon 64 3500+
Nforce 4 Ultra motherboard
4 512 mb DDR sticks
1 Samsung SATA drive
1 Samsung IDE drive
1 Seagate IDE drive
Zalman 7000-CU @ 5V
Two Fractal Design 120 mm fans @ 5-7V (Depending on application)
How quiet is it then? I'm positively surprised that it almost matches my P182 and I can always add more dampening material. This proves that any case can be quiet with enough modding.