I've been using an 8800 for a few months now, and have noticed that after shutting down for a while, the 8800 output is kind of hit or miss.
That is to say, sometimes, like right now, I can't get it to give any output at all. Both 6-pin PSU connectors are in, and I'm running a 650W PSU. The fans are spinning (more loudly than usual). I have an onboard videocard (6150 LE) that's happily allowing me to use the "Analog" source on my monitor. Switching to "Digital" puts the monitor into power-save.
Unplugging the DVI cable from the card at least gives me a "Check cable connection" message on the monitor. Because I can start up in analog with the onboard video, looking through the device mgr and video settings shows no 8800.
I should note that this happened when I first got the card, and I found some sort of workaround and had it work for months on end... I can accept that it may be shot, but would like to explore other possibilities.
Hm looks like I have no option in my bios setup to disable onboard video... Unless said option is labelled something counterintuitive. There was an option to flip gpu banks. Just toggled that to no apparent effect.