Well i ended up removing that fan anyway just to test the temperature. I also removed a bunch of useless stuff in the case to allow better airflow, and replaced the CD drive by an air trap
So far the temperature is sticking under 70 degrees. (but it's been running only for 1 hour)
I know that CPU is supposed to have a max temperature of 100C (but i would seriously be scared to let it go above 70-75) but i doubt my Power Supply could tolerate that... I don't have a temperature sensor so i can't know what is the temperature of it right now
Anyway, i'm still looking for a way to underclock or undervolt my Atom D525 CPU. Pretty sure it would help even if the voltage is already low (0.8250v).
I've been monitoring CPU load for weeks and it almost never goes over 25% usage so i would like if i could disable that 75% heat/power i don't need
Edit: CPU temperature keeps going up
Now it's around 75degrees