I'm skeptical about the GPU since a lack of a proper UEFI BIOS environment can and will lead to seeing only a black screen inspite of the system booting up to GUI(although you can't see anything). You may want to borrow a card from your friend/neighbor and see if the system boots up with display...then you can invest in the card of your choosing. On another note, how old is the machine? I'm trying to gauge the age of the prebuilt's PSU and it's effective power output.