So, I have an update which makes the problem more strange than before:
After I've seen that the integrated graphics works at first boot, I plugged the 7950 again, and I've seen that again I have to restart till I get view.
The next step was to take my previous card - geforce 260gtx, and plug instead the 7950. Second step, I took the 7950 and plugged into my previous computer, instead the 260gtx.
The results:
The older computer, with the 7959 started immediately without need to restart, and the most surprising that the new computer started immediatel too!
So, now I really dont know how to find the problem, but I have something on my mind:
Because I plugged the 260gtx, which doesnt support pcie3, it worked as pcie2.
In addition, the 7950, which supports pcie3, was plugged into an old computer with pcie2 only, so it worked as pcie2 too.
Somwhat I think, that something is wrong with the integration between the 7950 and the pcie3 in my motherboard.
What do you think?