Install SAPPHIRE HD 7970 on DELL XPS 8700

Hallo 
I need help Please 
i have a xps 8700 (working fine With GTX 650 TI ) 
I try to install a sapphire HD 7970 Graphics card. 
i get power and fans running But i have Black Screen!! (No Peep).
after around 20 Sec , the Computer Restart once and still nothing Black screen!!

*i have Thermaltake toughpower XT 775w power supply

I try the Following :
1) i put back my old Graphics card and uninstall all the old drivers.
2) i login to the BIOS and set Secure Boot to off.
3) i change boot from UEFI to Legacy .
4) i put the HD 7970 back and start the computer .
((( AGAIN NOTHING !!!))) ... Black screen and one time restart after 10 to 20 Sec !!

My BIOS Ver. is A11 (Latest i think)
Windows 10 64bit
CPU intel i7
RAM 12 G


Any Help will be appreciated

Thank you 
Ala