Here's a repost of my solution to a clean case....you could go further by sealing around the cards with silicone rubber, same thing around the drives. The front door of the case would help alot. The fan filters could be replaced often....maybe even improved.
If you're overclocked, or run hot internal devices, this case is probably not for you. I'm running a celeron 2.0 on a Gigabyte 8PE667 Ultra and I don't do much if any gaming. I've got a bad dust problem in this room (living room). Two large parrots....dusty, fly around, etc. I've got a large HEPA fan going 24/7, but there's still plenty of dust. Here's my solution....
Most cases aren't configured for a positive pressure setup....this one is. Antec SLK2600AMV, available at CompUSA with an Antec smartpower 300watt P/S for about $70. It's rather heavy, steel, with a rear port for an 80mm fan, and the same size front intake. It also has a tight-fitting front door that covers the drive bays.
The one thing that makes this case so special is the room around the fan ports. There is enough room to fit good intake filters. I use pleated paper filters designed for lawn-mowers...they trap dust much better than those thin foam filters you usually see. (Briggs and Sratten #5028) I attach the filters to the case with carefully fitted double-sided tape. Both these fans blow into the case (positive pressure). Only the P/S fan blows out. The two case fans and the CPU fan are thermally controlled 1500/4000rpm PCToys CrystalMax. Their speed/noise is minimal....usually run around 2000/2500rpm. Of course under load, their speeds go up.
The case requires just a few mods...no cutting. I closed off all the vent holes from the inside using duct tape.....except the two case fan ports of course. I lined every accessable place with acoustic foam....including the rear of the front case door. Very little/no air enters this case without passing through the two case filters. The weight of the setup, the acoustic foam, the variable speed fans, and the very tight fit of this case, make the whole setup very quiet. The positive pressure and the good filters make it very clean.
I'm pleased with the final result. It's a pretty medium grey metalic.....I've made no external mods. It looks stock. At an ambient temp of 22.9C with the computer idling the CPU temp stays around 32C. Under a severe load the temp has gone up to 42C...never seen it higher.
CPU....Celeron2.0
M/B....Gigabyte 8PE667 Ultra
Ram..512mb samsung pc2600
CPU fan/heatsink.... PCToys QuietMax 5700 using a CrystalMax Thermally controlled variable speed fan...With a scoop that faces the rear intake fan.
Two maxtor 7500rpm hard drives.
PCToys rounded cables
ATI Radeon AIW 7500
Scsi card
Pci modem
Firewire card
Making a clean/quiet running computer is not difficult....but you need the right case to start with.
One more thing.....I installed a DigitalDoc 5 (with 8 temp sensors). Amazingly the hottest internal part of this setup is the master hard-drive, which is at the bottom of the case with the front intake fan blowing on it. It usually runs hotter than the CPU......I put a small fan right over it that turns on at 37C(controlled by the DigitalDoc). It always turns on when the system is under load.
Here's a sample of the eight measured temps after the system has idled for two hours....ambient 20.9.
CPU...................31.4
Ram chip...........33.3
Chipset.............33.6
P/S output........29.8
Video card........31.5
Case temp........29.9
H/D master.......36.9
H/D slave..........29.0