Hello, I have very weird issue. I've been using my system about 1 year ago and everything worked fine. I had an PNY GeForce 7900GS 256MB PCIe x16 and then, i received my 8800GTX, installed it booted but no signal! then i moved it to the other available PCIe x16 and nothing happened. Installed back my 7900GS it booted! Installed the 8800GTX in other PC, it worked perfectly!!
Another weird thing is that randomly it boots (using the 8800GTX), with artifacts, and only loads windows in Safemode.
4GB (2x2GB OCZ Fatality ed ddr2 800)
AMD Athlon 64 X2 4600+
ASUS M2N-SLI Deluxe BIOS 1701
Windows 7 / Creative SoundBlaster Audigy2 ZS
Uninstall the drivers while the 7900 is installed, then run driver cleaner. Then remove 7900 and install 8800. Try that and see if it helps. Also do you have any overclocking software installed? Ntune, or Rivatuner, or anything like that? Uninstall them as well before installing the new card.