>That may have changed in recent years, but somewhere around 20 >Mpx approximates fine grain film resolution.
Kind of digressing here, but if you are talking about 35mm film, I believe it was far lower than that - closer to 5-6 mpixels.
In film the practical resolution was not the grain size, but that it took a clump of grains to define a distinct tone. A D600 has a pixel size of around 5.9 microns. Grain size was around 2 microns, but the clumping needed for tonal definition was much larger, (30-40 grains).
There is an old article on this on Luminous Landscape, kind of interesting...