Abit NF7-M (with Zalman ZM-NB47J)
AMD Mobile 2400+ 35W (AXMD2400FJQ4C)
Zalman Reserator 1
SilverStone ST30NF fanless PSU
Samsung SP1614N (on foam)
Antec 3700BQE without casedoor, no fans running. (But a case is not really needed, the closet is like a big case)
2x 1.8 meter PS/2 extension cables. (For keyboard and mouse)
This system costs about $1000 and has no noise except the harddrive and reserator. No drilling is needed.
First I had SmartPower 350S PSU running at 55-60W load, but it was too loud so I put it into the closet with the closet door slightly open, I saw no difference in temps and the noise was cut in half. Of course the PSU fan needs to face the door opening so hot air goes out. Temps were about 43C CPU and 30C mainboard with the casedoor closed, with AMD Mobile 2400+ underclocked to 166x8.5 @ 1.1v (8rdavcore).
After I got SilverStone ST30NF fanless PSU I moved the computer out of the closet but the harddrive was way too loud (I didn't know it was this loud!!), so I moved it back again to see if it helped, but this time I removed the casedoor because I had no fans running. Now I couldn't hear the harddrive anymore but I could still hear the Reserator from 3 meters. I put a cloth under the Reserator so most of the base was in the air, it removed the vibrations transmitted to the floor, but there is still humming noise that I can't do anything about, I can still hear it from 2 meters in a silent room. Temps were 44C CPU and 31C mainboard.
Because I have a fanless PSU I can overclock the CPU without the PSU fan ramping up. The best seems 166x14 @ 1.575v. Temps are 48-50C CPU and 33-36C mainboard. Folding 24/7. PSU load is 85-90W.
My Xavi modem has a very annoying high pitch whine so I have it inside the closet too and can't hear it anymore. My APC BACK-UPS 650VA that I use for my gaming rig also has a slight high pitch whine (I turn it off before sleep), but my APC Back-UPS ES 500VA doesn't bother me.
Monitor cable was just long enough.
With SmartPower 350S PSU.
Pen holder thing, the middle part is for envelopes or something.