how much does an iMac weight

Check the "Tech Specs" tab from the link below.

http://store.apple.com/us/buy-mac/imac

the older imac g3

Between roughly 37 and 38 pounds depending on model. If you need the exact weight for a specific model, this link will get you there:

http://www.everymac.com/systems/apple/imac/index-imac.html