Have you tried clearing the CMOS and then loading Optimised BIOS defaults?
Yes i did...
Again, I rolled back to the old drivers, put 2 graphic cards next to eachother, the old one (working) in the primary slot, the other one in the secondary slot and still nothing. Furthermore, now it won't detect the secondary (problematic 5850) card at all...
Also, tried it again, just shoving the 5850 as a standalone, and surprise surprise! The same... blank screen, no post, no beeps...
I'm freakin clueless on this one...