I keep a PCI video card around just for these kinds of problems. I use it to cheak the bios for the appropriate PCIe setup. Make sure you put the heat sink back on carefully. I'm getting the feeling, that if it is a 6800gt PCIe card, did you plug the molex plug (power) into the video card when you fired up the system. That may sound like a dumb question on the surface but I have found this to be a very common occurance for new builders.
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.