For Prints you normally aim for 300 DPI, which at the full D90 resolution is approximately a 15"x10" print.
If you start cropping you lose resolution, and if you want to print larger than 15x10 the DPI will be less than 300.
Generally though, the bigger you print stuff, the further away you will be looking at it from, so you consider a larger print at a lower DPI to be OK. It just takes a bit of experience printing stuff out, and will vary with the quality of the image.
A side issue, if the image is 1,000,000DPI but is very soft, the image printing site will say the resolution/dpi is OK, but the print will still turn out soft.