I have a very similar problem with an EVGA 9400GT on Windows 7 64. My dual dvi monitors worked under Vista 64. With Windows 7 the second DVI port refuses to put out a digital signal. I still have no working secondary monitor under Win 7.
I'm becoming convinced the NVIDIA drivers are to blame...
You can see the extensive troubleshooting that I've been through by visiting the thread on Technet below. (different cables, monitors, every driver know to man etc) http://social.technet.microsoft.com/...9-8772397237de
There are more and more people with this problem it seems.... Could be time for an ATI card