I dont know if anyone has mentioned this, but have you checked the connection to your monitor? (in back of your monitor)
The amount of flummox's i have made with building pc's...
One time I even forgot to put the ram in the board, and then i wondered why it wasn't working
If there is no video output, you'll most likely need to supply power to your GPU.
Have you got the 8-pin facing the right way?
(i.e the clip on the front going over that little nub on the connector itself)?
It's not my problem anymore, i took it to the local shop, good guys and they know what their doing and they're going to take care of it for me, i just can't wait any longer!