Here's a thread I found when planning my aquarium. It spells everthing out pretty thouroughly.
http://www.african-cichlid.com/Structure.htm
PS: Even though you can calculate the weight of an aquarium filled with water, don't forget all of the rock you will be putting into it, the stand, the sump, surrounding furniture,... every once in a while people passing by, taking a look...(maybe a few when you're having a party)... Alot more to consider than just the aquarium itself. Irregardless, most homes are built to the above MINIMUM standards. However, if you are planning on an aquarium which exceeds the minimum standards, then please refer to your household insurance for guidance. ;-)
Riggs.