Poor quality DVI/VGA cable? Those high resolutions can stress them,
particularly ifs it's 5m or more.

Not, obviously not that. Please think this through. You read his post. Same
monitor, same cable, but went from an ATI X1600 to an Nvidia 9600GT. It's
one of two things, or a combination of both. Going from an ATI card to an
Nvidia by just uninstalling the old drivers via Control Panel - Add/Remove
and then running the Nvidia drivers onto the system can be a major issue.
You need to use a driver and registry cleaner to do a proper job when going
over from one side to the other. The *.inf files likely have not been
installed after the switch, and it's possible that he has his display set
at an incorrect refresh rate (70Hz vs 60Hz)