We often tend to think that printing an image at a higher DPI (dots per inch) on a smother sheet will result in a superior definition. This was more true in the olden days when a photograph had to be shot by a camera with a screen in front of it in order to create a piece of film, than it is now when the PPI (pixels per inch) of the digital original will be a determining factor. While “normal” halftones in book work would print at 133 DPI, an art book, on coated paper, would print at at least 150 DPI, and often 300 DPI. However, if your digital original doesn’t have enough pixels to support these higher DPIs, then there’s really no point in increasing the resolution. For online viewing, the resolution available on your monitor or tablet will be the limiting factor.

This post from 99 Designs explains the issues. Follow the link to Wikipedia for more on DPI.