Once the images are scanned and saved at a certain resolution it is difficult to get an image to a higher resolution. You should always consider how the image will be used before you start manipulating it.
If the image is to be printed on a color laser or inkjet printer, 300 dpi is adequate.
If the image is for printing it doesn't have to be a .jpeg. A .tif or .pcx might work better.
The .jpeg format has built-in compression that can be adjusted in Photoshop that can affect the printing. The more compression, the lower the quality of image.
See illustration: http://www.artistnation.com/members/...pegoptions.jpg
On-line images for a web page should be small enough that the user doesn't have to wait forever to see it. The battle between image size and quality is ongoing. If you are using Photoshop, experimentation will show you how to get the best size/image quality for your pictures.
Sorry about the lecture but the main point is; you have to experiment with the settings and evaluate the resulting image.
This site has good information about web graphics