I know what your parts are and I think you misunderstood me.
Start from the beginning.
First, leave the GPU installed into the first PCI-E slot, with the power cables plugged in to the video card.
Next, plug your monitor in to one of the ports on the motherboard. That uses the onboard graphics.
If your PC boots, let it go all the way in to Windows and see if Windows detects your video card. If it does, then your motherboard simply is not booting to the PCI-E card like I said before. If this is the case you can plug your monitor back into the video card and turn your PC on. Wait for Windows to boot up and after a while your Windows screen should come on the screen.
I have been saying all this with the assumption that you haven't been getting any video to your scree while using the 7870. That is why I have been trying to help you troubleshoot your PC. If this is the case, then follow the things I am saying so we can go through a process of elimination to figure out exactly why you aren't getting any video with the 7870.