Did some looking for info for $boss today as far as the relationship of megapixels, DPI, PPI and what is needed to print something “nicely” for a project he’s working on. In my search I found Resolution & Print Size which gives an overview of why you need a certain number of megapixels to get a decent printout.
If you want to make a good 8×10 print under the above standards, it will be best to have a 300ppi image with a size of 2400 by 3000 (8×300 by 10×300), a 7.2Mpixel scan.
Page 2 on the site has a handy chart as well 🙂
If you have photoshop, it’s also pretty easy. Open up your image and then go into Image -> Image Size. Uncheck the “resample image” checkbox at the bottom of the dialog and then set the “Resolution” setting to the DPI you’re planning to print at. Because you’ve unchecked the ‘resample’ option the image dimensions won’t automatically resize for the new DPI, but the width and height scales will automatically adjust to show you what the size of your printed image would be. Here’s what I mean.
At least, that’s how I understand how it works.