Here are some initial tests of the Nikon Coolpix CP4500 Noise Reduction (Dark Frame) feature using a wide field view.  All shots are looking west over the City of Victoria, so some light pollution is to be expected.  Original images were 2272x1520 pixel in size.  Clicking on the thumbnails below will display an 800x600 portion of the original images.  No down-sampling or other enhancements were performed on the images.

All images below have the following attributes:

Effects of Exposure Duration on Noise Reduction

Effects of ISO Setting on Noise Reduction

Noise Reduction: ON - Exposure Duration: 8 secs

Conclusion: From the empirical evidence above, I would be comfortable using ISO 400 with Noise Reduction ON to reduce exposure times.  I also wouldn't hesitate to use ISO 800 should the need arise, since with Noise Reduction, this setting still results in an acceptable degree of image noise for short duration exposures.