Photoshop's resolution setting is in ppi (pixels per inch) -- this is an arbitrary number which has nothing to do with how many pixels are in the image. It denotes the "size" of the pixel. DPI is a term that applies to printers, the dot pitch of the tiny nozzles -- they dither a mixture of primary colors -- typically 4, but up to 8 -- to fool the eye into seeing intermediate hues.
You can change the ppi in Pshop (remember to uncheck resample image) -- all this does is affect the size of the image printout. But you can over-ride the ppi setting in Pshop in the print preview diaglogue. Watch as you change the image printout size and you will see the ppi number change. The default 72 ppi or "screen resolution" in Pshop is an obsolute convention (dating from the original Macintosh small screen). If you have a screen that say measures 20 inches wide and your monitor resolution is 1920 by 1200, then you are looking at 96 pixels per inch. You can do the math for any other screen...
Since most printers dither or mix dots to make a color, you generally don't want to send an image that has a ppi resolution more than 1/3 the dpi pitch of the printer -- extra information will not help the printed output. I typically print out large format images (say 40" wide) at about 100 ppi, or 4000 pixels wide and they look fine. Of course if you get real close and use a magnifying glass you can see the individual pixels and even the dots. But generally large images are meant to be seen from a distance at least great enought to see the whole image. Billboards, for example, are typically printed at 9 pixels per inch up to a maximum 36 ppi.
Paul
