Thanks for all your advices.
I negotiated to have one single machine running. It is purely symbolic. This will be planet MARS the god of war against the heat .
I am waiting now to have a specialist is ventilation and cooling to come and study my case.
I will keep you posted.

If you are running Vista or Win 7 you could underclock the systems (Power Options). In itself that would half the power consumption and substantially and noticeably reduce both heat and noise!
If you use water cooling instead of fans, might it be possible (secure) to put the reservoir outside? Bolt it to the ground or wall and use tubing to bring it in through the window? Basically most of the CPU heat generated would be released outside that way.
http://www.gadgetgrid.com/2008/02/14/computer-water-cooling-reserator-1-v2/
PS. I have the same problem; 32deg C in my office. My solution may eventually be to consolidate several GPU's into one system and do away with a couple of dual core systems for a quad or 6 core system. I guess there are a few of us with similar problems.
Last year I hooked up a 12cm case fan to draw cool air into the room and just stuck it on the window frame - it helped.

Tubing and outlet fittings for tumble dryers might be of interest?
http://www.espares.co.uk/datastore/ProductImages/610576.jpg
http://www.espares.co.uk/datastore/ProductImages/726933.jpg

Might dampen the noise somewhat too.

I think you would be better off running 2 systems at normal speeds because, when you overclock you use more electric and generate more heat. The amount of heat generated increases further when you increase the voltages. While an i7 can overclock very well, it also uses a lot of Watts. Two systems with normal speeds will make less noise than one heavily overclocked system. If the two systems are underclocked they may still do more work than one overclocked system, but produce less heat and noise.

There are 4 things to consider; Work, Noise, Power, Heat.

Work done - If a task takes 2h on your OCd system (say 4.2GHz), and the system uses (340W for example) then if you clock two machines to run the task in 4h (2.1GHz) they should do as much work.
Noise levels

Noise - Two underclocked (UCd) systems should make less noise, unless there is something wrong with the system design.

Power consumption - Two UCd systems might use about the same power (say 170W each) as one heavily OCd system (340W). Perhaps slightly more, and possibly slightly less. It depends on your original OCd system - the more you OC the more power is used relative to performance gain. At a certain point it starts to become very expensive, in terms of performance gain per Watt used. My guess is that this figure is about 4.2GHz (but it is just a guess, it could be 3.8GHz or 4.8GHz and depends on other system factors).

Heat (most important) - The best way to measure the heat is to use a thermometer and check the room temps. Perhaps on Sat for one OCd system and on Sun for 2 under clocked systems. This will inherently take account of your surroundings and the ability of one window to cool the room.
Although you could also measure the system and CPU temps (at various times of the day), that would not tell you how it actually effects the room temps, which is really what you are concerned about. It might be the situation that 80deg C heat blowing out of one system causes the room to heat up whereas 50deg C heat from 2 systems does not - even though the 2 systems use slightly more power.
Have fun,

Ganymede is now ready (uses my old i7 975X) but cannot be put in orbit due to my power/heat problem. I have found a solution with Deimos who has been put in another place in my home and can run 24/7 without heat/power issues. By week end unfortunately Saturn will have to be deorbited. It was active over the last week due to a steep fall in external temperatures which allowed to re-activate an additional machine. But now temps are on the rise again.

The status of the Solar System as per today is:

Ganymede (975X): de-orbited (off)
Phobos (980X): de-orbited
Neptune (980X): de-orbited
Mercury (980X): de-orbited
Terra (980X): de-orbited
Ceres (980X): de-orbited

Jupiter (980X): slow orbital speed (intermittent)
Haumea (950): slow orbital speed

Uranus (980X): in orbit (active 24/7)
Pluto (980X): in orbit
Venus (980X): in-orbit
Eris (980X): in-orbit
Deimos (980X): in-orbit
Saturn (980X): in-orbit
Mars (980X): in-orbit

Makemake (950): not created yet
Europa (950): not created yet
Io (950): not created yet
Callisto (950): not created yet
Charon (950): not created yet
Pallas (950): not created yet
Vesta (950): not created yet
Toutatis (950): not created yet


Speeds are all 4Ghz for 980X and 3.5 Ghz for the 950 and 975X.

I have in stock eight i7-950 CPUs with their respective coolers. These are future rigs not build yet but which will be built over the next few months. By end year when my WCG Data-center project will be finished there will be a total of 23 planets/satellites/asteroids.
The total cumulative points production capacity of the final system will be around 1 Million points per day.
The total cumulative points capability of todays existing system is around 710'000 points per day or 71% of my planned system.
Against all odds am still managing to contribute daily around 300'000 points or 42% of my existing capability. In the hot summer days it may fall to 200'000 points or less.

It is clear that I will never be able to run the whole system in my home. I am now looking for places were to have my system installed. I have some interesting options open. I will come back on that when a solution will be found.