I'm sorry, but where do you get that crappy idea?
I can really recommend AS5, it is easy to apply and my CPU (i7920 in combination with a mugen 3) is only about 35 degrees celcius with an outside temp of 22.
Edit: I just saw you are going for a HAF912, I got that casing this summer. Awesome thing, however, If you plan on putting a fan on the side door, take one that is low in rpm. I bought a 1700RPM 120mm but it turned my casing into a vaccum cleaner. I swapped it with the fan that was connected on the back of the casing, which runs on lower rpm and that reduced it. But that is seriously the only fan-place in that casing that is making a lot of noise.
I do however reccomend buying a 200mm fan and put it on the top roster (you can choose 2x 120mm or 1x 200 mm for that roster) I got the
Cooler Master Megaflow on there and itis really quiet.
Also, I only got one fan with my CPU-cooler and I placed in it in Push, since the casing already comes with an inbuild fan on almost the exact same hight as the CPU cooler.
Also: your HDD's will be running nice and cool btw, mine are 25 degrees
Another also: What HAF912 do you have? Since you are from germany I find it highly likely that you have the 912plus, since that is the european edition, but do note that there are 3 different models which are slightly different and are all called 912. I also have the 912plus and like i said, it is awesome (and silent as long as you manage to find a super silent fan for the side or put no fan on the side at all)
oh, another Also, they keep coming!
You said you have a fan-slot on the bottom, this is not true, this is the fan-slot that your PSU will be using. Your psu will suck its own air in and blow it out through the back, thus not intefering with the air in your case itself.