I have a similar problem with my old P780 Dell Trinitron monitor. I just updated to a EVGA 7900GT card, and it won't recognize my monitor is there. Weird thing is that when I installed Win XP for the first time, it worked fine with the default windows video drivers. It wasn't until I installed the newest NVIDIA drivers that my monitor loses signal and goes into standby mode. I started going back though the archive of NVIDIA drivers that I had saved thinking it was a driver issue. Still no luck unless I uninstall the drivers completely and it works fine again. Something else weird, if I start the computer with my wife's Samsung LCD using the same DVI/VGA converter, windows recognizes the Samsung monitor immediately. If I then unplug the Samsung, and plug in the Dell after windows is up and running, the Dell monitor will turn on and work normally. If you then go to the NVIDIA display properties, it shows up as a "default monitor (disconnected)".
I also thought it might be an issue with the resolution/refresh being out of my monitor's range at first, but I have it set as low as it can go and still have this problem.
I also tried getting the monitor drivers from Dell for the P780, and installing them, but to no avail. So currently, I am using my wife's LCD and she is using my CRT, but I would really like the Trinitron back.
I've tried using different DVI/VGA converters with no luck. I know none of the pins are bent, and the monitor works with my old 6800GT card with VGA port.
I know there are 3-4 pins used for monitor recognition, and I am wondering if they aren't actually wired to anything from the plug to the monitor. The DVI-VGA adapters are the ones that came with the card.
Any ideas other than buying a new monitor?
Relevant system specs:
ASUS M2N-SLI
EVGA 7900GT