It would be really helpful if you could determine to the best of your ability what the actual components in your system are, and fill in your system specs (under UserCP>Add System). Putting the info in a post (or a link to the makers website) is not convenient because then people have to go find it all the time.
It seems unlikely that the problem is that the power for the card is inadequate, esp. given thats a 780W PSU. It doesn't take that much juice to fire up a video card ... if you were having a power deficiency, it would almost certainly not manifest itself until you ran a 3d game, and what would happen is that your system would just shut off, like you hit a switch.
BUT ... you are using 'native' PCI-Ex cable off the PSU, not the molex adapter, correct? ANd 9800GTX+, that's a single 6-pin connector, correct? If not, are you plugged into both the jacks?
More questions: What type of cable (dvi, vga, etc) are you using, and are there any adapters involved? Have you tried the different output jacks on the card?
When you say no display, does that mean you cant' even see a POST, or just that it shuts off whne you get to Windows, and if the latter, what OS are you running, and how old is the install? If the former, does your monitor light change as though there's a signal at all when you turn on the PC, or just stay entirely dead like nothign is connected at all?
WIth the card in and powered up, but the monitor plugged into your onboard graphics instead of the video card, and the bios set to favor the onboard gfx rather than PCI-Ex slot, can you get a display and get into Windows, or no?
If so, check in Device Manager ... does the 9800GTX card show up under 'Display Adapters'? What happens if you try to install teh forceware for your new card ... does it work?