Pixels per inch - how many to give to the printer?

There can be a use for the scale information in an image, but this reply slightly misses the question, which is, as much, how many pixels is it worth putting in the original image? Or, what resolution should I use to match the printer's (and the paper's) capability? For instance, I can print synthetic images with 1000 pixels across on 6" x 4" (150x100 mm) paper. At this level (ie 167 pixels/inch) on my R800 I can see 'square' pixels (with a loupe, admittedly!). If I go to 1500 pixels across (ie 250 pixels/inch), I can see pixels, but they are starting to lose their square edges. Now, I could generate (or photograph) images 30,000 pixels wide, but it would be utterly pointless on 150mm wide paper., because we're now talking 5000 pixels/inch which nothing can print. As was said, the printer drivers generally manage the pixels/inch but it would be nice to know what it's worth giving them, so that we can tailor the size of the original image to suit. What is a realistic number for an Epson R800? Thanks.
If the printer does X pixels per inch, there is no reason to have more. I believe a normal color print (not negative or slide) is around 200-400 pixels per inch. You may find that pixels per inch (PPI) is not the same as dots per inch (dpi) on printers.