so you were able to use this PC fine, up until you replaced the graphics card?
I may have some suggestions to try;
1) Try another PCI-E Slot, no matter if it is running in X8, it can give an idea if it is due to the slot.
2) What pin power cables have been used to power the card?, The card may currently have the incorrect pin connections being utilized, have you got 2x6 pin power cables currently connected?
3) What PCI pin outputs does the power supply support, if not the right outputs, this will explain the black screen occurring.