i had the same problem but it was not my card (8800GT) it was my pci-e 2.0 slot 1 .... it was very temperamental tho .... any time i shut the power off (unpluged, no light on mobo) and after installing new hardware new fan, hdd ect.. then turnd it back on, the card would not work (no power) but if i used an old card (ATI 2400 pro) every thing worked ... so i shut down left power on (did not unplug, light on mobo on) changed the card back to 8800GT and all worked ... dont know what it wrong with my mobo (asus m2n-e sli) but that is the only way i found to fix it.
btw i dont think its power problem as i have a 800w psu and if you didnt have enuff power the card would not work at all proboly just get that beeeeeeeeeep telling you that there is no power.
try the card in a friends pc