When I went from a 9800GT to a HD5850 I had the same initial issue. IF you did not uninstall the Nvidia drivers first, you will not be able to see the display until windows installs the generic driver. What I did was let the system run for a few minutes with a blank screen; enough time that I knew it was installing the drivers, and then manually reset the system. When I turned it on again, I had the windows display drivers. I am not sure if this is the same issue, but switching back and forth between Nvidia and ATI cards can cause this to happen. If you are not using both cards, make sure 8800 gts is removed and the system is given enough time to install the generic windows display drivers.