Hi!
Thank you for all your support!
Today I just had enough of this mad experiment and went to buy a Gigabyte X99 UD4 motherboard, since all of my previous builds (4-5 of them) were only made with Gigabyte and I had never had problems installing a hackie.
I have followed this guide:
http://www.tonymacx86.com/yosemite-desktop-support/151067-5960x-yosemite-install-guide.html)and everything went smoothly with my old 9600 GT with nv_disable=1 flag ( otherwise it loads a little bit, then the screens are becaming black. )
Anyway, in Yosemite, everything is moving reaally sloow, the graphics are very laggy.
Only a single problem encountered: asfter the installation on hdd, trying to erase the kernel from the HDD and also copy the kexts from the usb stick failed.
So I just put the hdd in my other hackie, patched the kernel, copyed the kexts, repaired the permission, put the hdd back in my new build and everything was ok.
I also tried to use a GTX 760 Graphics card that I have borrowed from a friend and the black bar is not loading up, only the apple logo appears with an empty bar underneath it. I have tried nv_disable=1 and also nvda_drv=1, none of them worked. ( only the apple logo appears, with an empty bar underneath it, then, after a minute or so, the screen are going black).
Anyway, that prooves that since I made from the very first trial that I was not that wrong as I thought...and, most important, that wasn't doing something that wrong...
Another difference was that I have made the bootable usb from my other hackie, on Yosemite. Until today, all my USB bootable sticks were made on my hackie laptop which has Mountain Lion installed on it.
So... at this point, I really believe it is the Asus x99 Deluxe's latest bios which causes problems. I will try to put an older bios on that one and will try again.