If you are printing from a home printer then you want PS to do the resampling of the image and 240 to 360 ppi is about right. Some say for best results that number should be a number that will divide evenly into your output DPI. ie 1440 divided by 4 = 360 or divided by 6 = 240 but I'm not so sure the eye could tell the difference. I use 300 dpi.
If, on the other hand, you are sending the image out to a processor leave the resampling to those machines as the are far better than the resample engines in PS. The trick here is to provide as much of the original image to the processor. In other words resize the image (Image>Image size. . .) uncheck the Resample image box. Now by resetting the longest side of the image both the short side and the resolution will be resized to fit without losing pixels. If you are going to crop the image you will be cutting something from the image. Leave the resolution field blank on the crop tool menu and the remaining pixel count will be stretched (or shunk) to fit the desired size. Example: assume you have an 11 meg pixel image a 4x6 yields a 400 ppi while at 10x6.667 (keeping original ratios) it will be 240 ppi. By cropping you cut unwanted pixels redistributing the remaining pixels according to the indicated size. Confusing I know but don't worry about it too much. Just know that you are delivering as many of the original image pixels as your crop will allow. What ever the pixel size of the image will then be resampled by the processor to fit the image and resolution of that machine.
