I bought a used Hd 5770 graphics card. The guy tested it in front of me with his 2007 Mac Pro. I bought the power cable on Amazon to connect it with it. I just did installed it this weekend but it can't load it pass the apple logo. I tried putting back my old Gtx card in but it won't work if the hd 5770 is also connected. When I took the 5770 out then the old card works. Pls can some one help me. I payed 160 for the card.