Alienware Aurora R4 problems with GPU upgrade

Hi guys 

I have Problem with my Alienware Aurora R4 GPU upgrade I want to go from 560 GTX Ti to 770 GTX

But my system doesnt want boot when I plugged in the 770 GTX, Only show Bios starting screen and I couldnt get to boot menu or bios menu or doesnt even react to clicking F2 and F12, then after 2 min shows only black screen without nothing, then I tried to put the 560 GTX Ti back and the system load normaly, Then I was looking on wiring the difference was only in that 560 GTX Ti got (2x 6 pin Power cable needed)

770 GTX got ( 6 pin and 8 pin power cables needed so I plucked basicaly the 6 pin in first slot then the other in second slot and the 2 pin additional cable which was next to the second 6 pin power wire).

System:

Windows 7

CPU : Intel 3820

RAM: 4x2GB 1600 mhz

HDD: 1TB 7200 rpm Seagate

PSU: 875 w

GPU : 560 GTX Ti (working) 770 GTX ( doesnt want to work )

Thank you for leaving me a feedback or answers to this problem I appreciate that :) .