From: http://www.photokaboom.com/photography/learn/printing/1_calculators.htm#Print

Calculators 

If you don't have enough pixels to make a print at your desired resolution, add some more pixels.

This is called resampling or interpolation.

In Photoshop Elements, do the following.

1) At the top of your screen, go to Image > Resize > Image Size.

2) Make sure Constrain Proportions and Resample Image are selected in the lower-left corner of the window.

3) In the menu at the bottom of the window, change Bicubic to Bicubic Smoother.

4) Highlight the width value, and enter the value you need.

The value you use is the width (horizontal) value from the calcultor.

The height value changes automatically when you enter a new width value.

5) Click OK.