Since all three of these cameras are nominal '3MP' cameras, we can predict that results from the D30 will be better than those from the Coolpix 995, which will in turn be better than those from the Xi, assuming we use a lens of the same optical quality on each camera.
For the same megapixels and lens quality, a larger sensor gives a much better print. The author doesn't mention light sensitivity, but I believe the larger sensors (esp. CMOS) also capture more light.
That should be our plea to camera vendors. Keep your $#@ megapixels (6 is enough). Give us bigger sensors and better light sensitivity (ISO 800 equivalent with low noise).