Hi, Maybe someone here can help me. I have a dual CPU 8 core Xeon mac pro. 16 GB mem. I just bought the ATI R9 270 OC card edition from gigabyte. I also bought the 2 special power cables that I needed (from main board to 6 pin power cable to power the card). Installed my card, connected the power cables, but nothing,... No screen, even after 5 min, nothing happens on my screen. Anyone with some suggestions? BTW, yes I installed a 64 bit boot loader, and I am running mavericks.