Update: Finally I had the chance to test the system with updated BIOS, bad luck though.
I've inserted the 6850, went to BIOS, theres a pretty good Storage browser, it shows the MoBo and which slots are used and which slots are free,
and it showed that my PCI-E slots were empty, but i've just inserted the 6850, I thought I might have messed it up (how can you mess up seating a card?)
So i've reseated the card, went to BIOS again, Storage browser, and the card shown up! I've closed the bios then, the PC restarted and as if a squadron of WW1 planes were flying over my head the loud fan noise came again, this time without black screen, because I had my integrated graphics still plugged in with a VGA cable.
I've tried to find out where the noise came from, it's hard though but I think it came from the bottom part of the PC, so either the GPU or the ?PSU? fan. I've restarted the system, same level of noise and I'm so scared something gonna happen to the PC I've shut it down again.
One more restart, went to the BIOS, Storage browser and the GPU is nowhere to be found. Since 2 of my GPUs cant go dead at the same day I kinda think its a faulty PSU.