Always expect all sorts of crap when you upgrade Windows.
Clean install from scratch(boot from 8.1 DVD or usb, format c: and install).
But just in case, have a backup plan ready before you start, that is a Windows 8 DVD and a valid product key in case something got messed up when installing 8.1...
I installed Win8 more than once before and I have GeForce 680GTX and never had a problem. I usually uninstall the driver windows installed for NVidia and then I install the latest one from NVidia site. Never had an issue
My bad ! I am sorry, it works !!! the additional link at the end of your post explains things in a wrong way, but your method works !
Thanks man and sorry again..
Now the problem would be how to clean install 8.1 with my 8 key !