If it is a bad card, then you will be a little closer to a RMA. I hope not. Another thing is, do you have the monitor plugged into the left digital out put on the card? I didn't have a post screen either with the 7800gs.
I know its weird, and it may mean nothing in this case but, just trying to help, in the excitement of getting the card, you may have forgotten something......
I had a similar problem with an ATI 9800Pro.When i used a dvi to vga adapter it worked fine
Also riv do you hear any beeps from the mobo?If you have a speaker.