My Nvidia 8800 GTS needs another VGA card to work !!

HI .. I am Tiger

I recently upgraded my pc . I bought the new i7 CPU with MSI x58 motherboard and a 850w PSU ..

After assembly . The PC starts but nothing on the screen ..
I installed the old vga card ( nvidia 7100 that does not need extra power ) and It worked after that I reinstalled the 8800 card and it worked , but if I restart the PC the same problem appears again and I have to do the same thing plugging the old card then remove it to make the new one work (even if I just remove the new card and install it again it will not work ).

So why is 8800 GTS card fails to start ??