How much cooling can I expect from a central air conditioner?
A properly functioning and properly sized central air conditioning unit should be able to maintain the temperature of your house at any reasonable temperature you select.  The amount of cooling available is related to the size of the system, insulation in the house, and finally outside air temperature.

From your description I would guess either you don't have a big enough compressor, or you don't have enough airflow through the house.  It's also possible the cooling element in the system has become dirty and no longer transfers enough heat from the air into the element.