Generally, a house is designed to take a lot of weight, newer houses are usually built to code to be strong enough to hold the weight of a car. Older houses are usually just made with heavy duty material, and if it isn't deteriorating, it will be strong enough for a lot of weight. Going to a common heavy object in a 2nd floor of a house, a filled cast iron bathtub has a weight of over 600 pounds in a very small area.
That said, it kinda depends of the condition and quality of the house build, and without an inspector its hard to get an estimate. You would need to find out how big the floor joists are and how far apart they are spaced. Unless you have an exposed attic area to see this, it is probably not even worth checking.
If most you stuff is against the walls, it will really help to prevent any floor sagging that could occur, but some heavy stuff in the middle is fine because the weight will distribute itself out between the floorboards. Throw as much as you can against the walls, especially the walls that make up the outside perimeter of the house, they are made to take more weight.
I think what you need to do is take the total weight of everything you are going to put in the room (including permanant fixtures, if any), and divide it by the room area. I believe the limit on a 2nd floor should be built 30-50 lb/sq. ft. I probably would only feel comfortable going up to 15-20 lbs. Certain things you just don't want to test the limits of.