Specs:
Abit NF7-S 2.0
Barton 2500+
Zalman CNPS-7000AlCu
1 GB RAM, 160 GB PATA 7200.7 (suspended)
Leadtek Winfast (5900 XT)
Zalman ZM400B-APS

... all in a black miditower, Compucase Ci-6A21.

Materials:

1 plastic 1.5 litre bottle, which first contained Ramlösa ("PET-flaska" for Swedes). I knew from before that a cut-off part of the bottle fit inside the heatsink, at the same time letting the CPU fan spin unhindered (hurrah).
1 piece of plastic ventilation pipe, 25 cm in length. Outer diameter 85 mm, inner diameter 81 mm. This was used as the duct.
PVC hose (diameter 5 mm) for lining the hole after the cutting.

Tools:
Tin snips
Electric drill w/ 8 mm bit
Hole punch
Utility knife

Procedure:

Measuring: Borrowed lipstick from my fiancée to mark the hole. Taped the lipstick (with its sleeve) on the fan of the CPU heatsink. When I closed the side panel, the lipstick wasn't long enough -- fixed that with some loose pieces of closed cell foam between the fan and the lipstick. Now I had a mark for the center of the blowhole. Punched the center.

Drawing the outline: I used a compass (compasses?), marking the diameter. I wanted a hole slightly larger than 85 mm so that both the pipe and the would-be lining would fit.

Drilling the center hole: I drilled the hole using an 8 mm bit.

Cutting the hole: I started from the center (drilled hole), spiralling out. Started with large cuts, finished with minimal (about 2 mm/cut) to get a nice and even edge.

Lining the hole: Cut a length of the PVC hose and sliced it along its full length to open it. Folded it inside out and put it around the hole. The lining serves four purposes:
1) Protecting me (primarily) and the duct from cuts
2) Sealing the small gap between the pipe/duct and the edges
3) Holding the pipe/duct in place
4) Looking nice

Adapting the size of the pipe to the HS recess: I cut the bottle off with the utility knife about 1 cm below the point where the diameter of the bottleneck becomes constant. I then slid this cut-off piece, pouring end first, into the pipe until it stopped. I now knew where to make the next cut, which was where the pipe and the massacred bottle had contact + 2 cm. After cutting the adapter to size, I taped it to the pipe using black electrical tape. Basically, the duct was now finished.

Fitting the duct. I pushed the duct into the side panel a bit. The fit is snug, so it stays firmly in place. I put the side panel back on and pushed the duct far enough so that the adapter piece slid into the recessed fin on the heatsink.

Turned the computer on. Adjusted the duct a bit. Checked the temperatures (drum roll please...)

Temperatures and voltages
Conditions:
200 MHz FSB x 10 (=2000 MHz), 1.65 V
CPU fan at 5 V
Case fan (120 mm Papst 4412) at 5 V
Ambient temp around 22 C

Idle, before: CPU 44 C, case 28 C, HDD 32 C
Load, before: CPU 53 C, case 30 C, HDD 33 C

After modding, same CPU frequency, voltage, FSB, and fan speeds:
Idle, after modding: CPU 38 C, case 22 C, HDD 39 C
Load, after modding: CPU 46 C, case 23 C, HDD 39 C

Overclocking a bit to 200 MHz FSB x 11 (=2200 MHz), 1.85 V
Fan settings as before the mod

Idle, after modding, extra OC: CPU 42 C, case 22 C, HDD 39 C
Load, after modding, extra OC: CPU 51 C, case 23 C, HDD 39 C

HOOO-AH!

Conclusions:
All fan settings the same, all temperatures except for the HDD were lower by a large margin, even after overclocking the CPU so that it yields an estimated 38% more heat. This is based on the assumption that heat increases exponentially with voltage and linearly with frequency -- the calculation was done as follows:

(New freq/old freq) * (new voltage/old voltage)^2 =
(2.2/2) * (1.85/1.65)^2 = 1.38281828...

The only drawback is that the HDD temps have gone up somewhat, since a lot of the air now enters the duct instead of at the lower case front, where the HDD is suspended. The temps are still very safe, though.

Edit: Moved the rear case fan to the front, providing the HDD with cool air. Did this after discovering that the HDD temp was 50 C after an hour of gaming; now it never exceeds 35 C, not even after gaming gaming followed by defragging.

The case temps went down substantially. This might be 'cause the temp probe is located in a suitable position, but also since less, if any, air is recirculated in the case.

And yes, I enjoyed it. And yes, I would definitely do it again.

And no, I don't have a camera, so no pics unfortunately. Too bad, since I'm really happy with my work. All that remains now is to cut off the duct so that it doesn't stick out of the side panel too much, and perhaps fit a filter on it. I'll save those things for a rainier day.

Too bad no pics. You succeded in lowering your CPU temps, which is nice, but what about noise levels? Every time I hear a case with a side opening, I can hear the noise from that hole.

I know those Zalman heatsinks are perfectly setup for a straight intake duct to the left panel. But I wonder if noise levels would be reduced with a ninety degree bend in the duct, and the opening in the rear?

Anyway it's obvious airflow in std ATX cases can be improved with modification....keep working at it.

Sounds like a job well done!

In my experience, straight intake ducting does work wonders.

I think a good position for these computers is on the left side of the user - then the noise is just as good as being rear mounted as it is facing away from you, plus it has the advantage of having no wall to bounce the noise back. It probably doesn't matter where the computer is positioned once all fans are spinning close to 1000 RPM or lower, as I have done.

Bluefront,

my plan is to cut off the duct (now sticking out ~10 cm from the side panel) and put a 90 degree bend in it -- I'm way ahead of you -- and possibly also a filter. The big question is which way the bend should face. BTW I haven't explicitly noticed any changes in noise, since there are other parts in the system (mainly the PSU) which makes more noise. I did adjust the duct so it sucks in enough air, but without creating air turbulence.

There is just a slow stream of lukewarm air exiting at the rear of the case, so sucking air in from the rear would save me some noise. As I said in the first post -- I'm saving that for a rainier day.

I was also surprised how easy it was to cut the hole. My tin snips weren't very 'Wiss'y, but even a klutz like me could do it -- and the result was almost without any jags whatsoever!