ok so if i got it rigth you´re telling me if i use the second 6+8 cable i have the complete 36 amp at my disposal except for the amp needed by the cpu and other stuffs and that psu can handle an hd 7950?
Yes, assuming the PSU is wired so that the 2nd PCIe cable is on the 2nd rail. Which normally would be the case.
The HD 7950 3GB requires 200W at max. That's apprx. 16.7A on the +12v rail, or 8A+ per rail. A 125W CPU (for example) will require 10.4A from those rails also. 200W + 125W = 325W just for the CPU and GPU. Other +12V devices won't be as demanding. If possible, you should try to keep your PSU at 80% or less. But not carved in stone. Remember, your GFX card will rarely reach its maximum demand of that 200W.
(most CPUs require less than 125W, but I used that example as a max)