Does the computer beep? If not, it my be your RAM, your power supply, your processor, or your mobo. Have fun figuring it out.
Or your keyboard, mouse, OS or solar radiation. Try reformatting your hard drive and replacing all ram first.
Seriously,
I also tried a different video card and monitor on this computer. Still no luck though.
So if 2 monitors and 2 video cards that are confirmed good don't work on this computer then you have a motherboard problem. Your video slot or the pci slot you're plugging the video card into is fux0red. The monitor gets power from the wall, not the pc. The video signal sent through the monitor cable is low voltage so unless you've got like 4 cd-rom drives and 2+ hard drives running you're probably good.
Edit: if another monitor will work with this computer and video card it might be the display driver or display settings that are bad/missing and keeping the screen dark. BTW, is the screen dark because it's in powersave mode and it's not getting the wakeup signal from the pc, or does it appear to be working fine but just dark screen?