Check your monitor to see if it has an input button to press to select the source input to be DVI now instead of VGA. Usually the source input connection on the monitor is detected automatically, but not always.
If that doesn't help, then try a DVI (monitor) to DVI (GTX 660) cable.
In addition, do you have the latest NVIDIA video drivers
Latest NVIDIA ForceWare Video Drivers Windows 7
Hope this helps for now,