I don't pay any attention to dpi when I save my rendering.  When I print out on photo quality print, it always looks great-providing I don't run out of ink.  As for what format I save the master in, its .tiff because of the mask (alpha channel)-then I can use it in VUE as a billboard-4 polygons render a little faster than .pz3 polys.  Sorry drifted off topic a little--Plutom