I have a 50gal upstairs on wooden floor boards and there is a visible bend in the ceiling in the lounge because of it.
It is the opposite way to the boards and against an exterior wall but it is still causing a sag, the house is only 40 years old.
I would suggest if you do go ahead with placing the tank upstairs you put a plastic sheet underneath your carpet where the tank will go, this means that when you do water changes etc. the innevitable spilling of water won't weaken the wooden boards as it would normally. Also, make sure the tank is placed against an exterior wall, preferably near a corner, this is where the floor is strongest.