As I (slowly) learn the realities of this, I've come to the following rough conclusions:


Good prints look best if they're printable at 300ppi. Large prints may be OK at 150-200ppi, on the assumption that no one is standing right at the print and staring at that level of detail.


Printers fit pixels to physical dimensions (so a 2400x3000 pixel image file printed at 8x10 is sized for 300ppi) and then those pixels are mathematically expanded to suit the dpi of the printer head (so a print head that prints at 1200x2400dpi would convert those pixels above into a 4x8 rectangle of dots per pixel).


With those conclusions, my life has become easier.