That's an easy one: On a web browser, all images are displayed "pixel by pixel" (one pixel in the image is represented by one pixel on your screen), so you may keep resolution at 72 ppi (tough this is more myth than truth)and be more concerned about pixel count, that is, how many pixels tall and wide your image is.
Just to give you an idea, the image behind this link http://www.zero-zero.com/fotosgal/fp37.jpg
is 50x70 pixels, while this one http://www.zero-zero.com/fotosgal/fg52.jpg
is 414x300 pixels (These are linked from my website). Try not to go bigger than 600 pixels on the biggest side to ease download times.
You will find numerous tutorials on the web that explain this extensively, so I won't repeat it here. Just do a search on "scanning tips" or some such and you'll find more than enough. If you're feeling lazy, I'm throwing one useful link for you to answer all your questions:
Hope this helps,
[This message has been edited by Photog Rob (edited 20 September 2002).]