How much can a 8gb or a 16gb SD card hold on the Nikon D3100?

I need a card for my camera, and right now there are a bunch of sails on the cards. I was going for the 16gb, how much pictures could that approximately hold? As well as the 8gb?

Too much.

Having such large cards will only make you lazy. You will probably stop copying your images to your computer at the end of each shooting day.

You need to be in the habit of copying all your image files to your computer at the end of each day and once they are safely there, formatting the card using the format feature on the camera to format your card so it is ready for your next days shooting. For that, you will only need a couple of 4 gb cards, one for your camera and one to carry with you.

This information is on page 187 in your manual.