I just assembled my first computer and its not displaying any image after BIOS.
The motherboard is a P5N-E SLI and i have two nvidia 8800 gtx graphics cards that are securely attached to the motherboard and are getting power.
I do however have to use a dvi to hdmi converter, because the back of my monitor which is a tv doesn't have a dvi input.
The computer did show at my uncles house just fine and it managed to show something once where i currently have it, but i haven't gotten an image since.

Are you sure you have a big enough power supply? What happens if you remove one of the graphics cards (they use a lot of power)?