for the unitiated, you can send any resolution to any printer and they can print it to any size for you, whether u want it stretched, or cropped, or leave white portions of unprinted paper.
as a general guideline, or at least this is what i do, i send everything for print at 300dpi in jpg. this is for photographs/collages. for offset prints (which i don't do anymore) i usually send thru a pathed freehand file, which is "resolution-less" (can print at any resolution without losing quality). but this doesn't work for pixel graphics (eg, photos, these will be embedded or linked in the document).
so, if you want to print S8R, size your image to 2400x3600 pixels, whether u wanna crop, stretch up to you.
to ans your question on whether the eye can see a diff, well my eye can't see the diff between 150dpi and 300 dpi. if you are really worried, just send thru a test pattern or something like that at different dpi, and see if u can tell the difference from the prints. bear in mind tho that test patterns are not real world examples, more of a technical test.
all said, 150dpi would be the lowest i'd go for photographic prints. (image quality like noise, sharpness and contrast would make a difference to apparent "resolution" but that's another story altogether)
