[airspirit]

This is an easy AND a hard task.

First off, there is no such thing as a perfectly silent PC. Your HDD makes noise, your optical drives, even your mosfets if you are attuned to that sort of thing (I can hear them buzzing ... my wife can't). The trick will be in removing heat.

First, there are fans that are nearly silent. Look at the Panaflo L model 120 mm fans. Those are so silent that you can't hear them from over a foot. If one was set up to blow over your innards (PCI/AGP cards, mobo) and vent out the back in a sound-deadening case, you wouldn't hear it. At any rate, the HDD would drown it out even if you COULD. My recommendation is this: Make one port on the top of your case blowing in, and one port in the front bottom of your case blowing in, and have an opening on the back by your cards for the air to escape through. This would aspirate all the doo-dads on your board so your components don't fry. It takes a very OPEN case to cool these without airflow. With an insulated case, you'll just have to deal with it, though if you run your fans at 7V they will be inaudible even with your head against them.

There are also kits (boxes) to put your HDDs in to quiet them up substantially. I don't remember where they can be found, but that will take care of the biggest source of noise left once internal aspiration is taken care of.

Naturally, you'll want your CPU, NB, and GPU watercooled. Additionally, read up on watercooling your PSU. Some can be done relatively easily, and some are extremely difficult. This may be something that you want to air cool with low noise fans (Panaflo Ls). That's up to you.

Make sure your RAM has heat spreaders on them. Since airflow will be severly borked, they need to spread heat as well as possible. Similarly, you will want a decent sink on your southbridge. Even though they don't put off too much heat, you don't want to take chances.

Parting it out, you will need the following:

1xCPU block
1xGPU block & ramsinks (important!)
1xNB block & SB heatsink (important!)
2xPanaflo "L" 120mm fans (add pots if you wish)
1xCustom block for your PSU (some HDD blocks work for this ... read up)
NxRamsinks (VERY IMPORTANT!!!)
1xMassive radiator (for external passive cooling ... you can't internalize this system)
1xQuiet Pump (there is a pump thread somewhere in here with suggestions)
1xLarge External Res (allows coolant normalization from temp spikes, since the rad isn't working at max efficiency)
NxHDD silencing boxes

Setup an external cooling system however you see fit, keeping the pump/rad/res outside of the case. You can probably build manifolds or whatnot inside your case for the various blocks, but keep all obstruction to a minimal, since you want to capitalize on as much airflow as possible. The res should be at least 1 gallon in size to help normalize temps after the blocks and to aid in filling this thing (you will NEED it in an external system). The rad should be as big as you can find ... a Lytron 6320 can be snagged from EBay for not much anymore, but they are 2x1 FEET in size, but should be able to handle your system as described fine without fans if it is in an open area (I.E. not against a wall or something). Your pump should have at least 200 GPH @ 10 feet of head to handle the blocks and the rad if you use that rad.

Put all that together, and you will have an inaudible system that will be very reliable.