Underclocking an athalon 1.33 GHz to 600 MHz in order to get some peace and quiet, and yet are putting in the loudest hard drives known to man? Double bonus points for the massive underclock, but that hard drive really needs to go. If it were a matter of price... Sell me the silverado, use a panaflo on a stock cooler (1.33 GHz to 600 MHz? Does that even need active cooling?), and get yourself a Barracuda
Single CPU, Single HDD... Can I assume a single CD-RW drive? With those low, low specs you could probably use a much smaller PSU than 300W. You could probably find a disposable 150W OEM ATX PSU and an old 400W AT PSU at a swap meet for 20 dollars, then swap the heatsinks from the 400W into the 150W drive. Either way, your power requirements are probably going to be floating around the 100w level, where most 300w PSU's are inefficient. Plus, if you decide to go fanless on a 150w PSU and it doesn't work out, you're only out 10 bucks
. Do the math yourself, of course, an equivalent P3 system would konk out at about 75w... with that CPU and motherboard you may be adding 50w or more.
You have specs sheets, I'm sure. High-quality (heavy!) low-power cheap PSU's exist, and are a great thing for silence.
Have you thought of making a double-hull case? Not only would it be incredibly unique, but it would help with wood's inherint acoustic transmission and reflection properties. Remember, they make guitars out of it for a reason... I wouldn't make a wood case without some sort of absorption material, and I would go out of my way to make the insides as irregular / pitted as possible to diffuse reflections. Just the insides, of course.