COMPLETELY quite. That means he's going to have to remove every stinky fan from the PC...and amen to that!
The obvious thing to start with is water cooling. With either a P4 or Athlon64, about any watercooling type will cool it at least as effeciently has a HS and a fan. I haven't seen a passive HS for either of these, nor would I put much faith in one.
The next is a look around the case for other fans. Does your board have a fan on the Northbridge? If so, consider a water block for that as well. My newer Asus boards have large passive HS's on the North and South bridges, so I've left them alone for the time being. But I'll bet you can skip the Waterblock and get one of these massive HS's it a fanned-HS's place.
Then you have the video card's fan. While some are not loud, it still makes a noticable noise. If you want to make it silent and are interested in water, then get a block for that as well. But again, there's a couple passive coolers out there that you can install and avoid water.
Then you have the fans in the PS. Antec has a PS that's fanless called the Phantom350. Coolmax (?) also has one, but Antec is a trusted name in powersupplies.
Now, in normal watercooling rigs aimed at performance, there's usually a radiator with a fan on it. But with the soul aim at silence and not OC'ing to the hills, you can run pretty well without this on all the time. In the Viperlair's review of the WACC reservoir
, they ran their rig for a while with the radiator's fan off. Give that a read! I'd still install a radiator with a 120mm Panaflo for when it gets hot, but it's nice to know that you can "rig for silence" a PC.
Now all that's left is the case fans. But...do you really need them? I suppose you just about have to have one at least to get air moving over the HD (even if you put a passive cooler on, it still needs air moving). SO, perhaps leaving the fan on the radiator on isn't a bad idea.
I have one 120mm fan on a radiator, and one on the HDD cage (panaflo's) that run slow and quiet, but still have great flow at the standard speed. I have also added a 3-way switch so I can knock them down to half-flank or completely off. I intended this in the design to enable the water to circulate and cool even when the PC is off (half-speed if it's ever off or prior to a long shutdown) but it's come in handy when I found out that the half-speed setting is just fine while it's sitting there in an idle state (not folding@home yet with it, so the system cools down at an actual idle).
I can't run it with the fans completely off like I could if I had a passive rad/res like the WACC...but it's a nice thought. But at 32db (that's what THEY say), it's hardly noticable even on full tilt. This is why I swear by slower speed 120mm fans. They're just normally quiet.