As Kurt implied, it is actually pressure that is more of a factor than total weight. The pressure exerted will depend on the type of stand that you are using. Kurt seems to be using a cabinet type that has a full bottom - some do not! A cabinet that has an open bottom will have a greater pressure exerted over a smaller contact area.
An iron stand would exert a heckuva lot more pressure because the feet are a lot smaller than the bottom of the whole tank! The pressure exerted by the tank on the floor using an iron stand would be along the lines of 1500 lbs./sq. in. In this case, you would want to make sure that the feet of the stand are directly over the joists or as close as possible so as not to punch through the subflooring.
But you're probably using a cabinet stand anyway, right?