Jambonot :
Firstly thanks a lot for your answer !
Why is a quad channel RAM better ? If I take 16gb in a dual channel, I have some room left if I want to put more RAM, no ? Is there a difference in the performance ?
Do you know a better cpu cooling ?
I shall keep the 4960x, even if the 4960 is just a "little" more powerfull. I need it.
What do you think I could change to make this build even more powerfull ?
You could make the build more powerful by OCing that chip; however, to do that, you need good cooling. That CPU cooler is a top of the line air one. But that just decides how you get heat away from the chip into the case - you need to think about how you get heat out of the case now. I'd suggest you plan your intakes and exhausts. You should also consider using water over air, but thats on the outside of my personal experience. Also, I'm not familiar with that MoBo, but you should to make sure that the CPU cooler you're using doesn't block those RAM slots' head spreaders. Thats an advantage water blocks/water cooling have - less weight on the MoBo, and less space.
RE RAM - if you buy four sticks of matching RAM, than the bus on that MoBo can effectively quadruple the memory speed. So buying 4 sticks which total 16GB should be faster than 2 sticks totaling 8GB which only uses 2 channels.
Also, think you mean the 4930 (not 4960).
Finally, I am ignorant on the whole "running Apple OSs on non Apple hardware"; you should listen to COLGeek...