The only acceptable ways to replace a two prong receptacle with a three prong are: To install a gfci receptacle and label it as having "no equipment ground". Or running a new ground wire to the existing grounding system within 5 feet of the entrance of the grounding system into the house.
The problem you have is that you only have two wires running to the outlet box. You have no existing equipment ground. Unless you have a steel conduit system in place that is totally intact and connects to the breaker panel box, the self grounding receptacle will be of no benefit. The only thing that they do is connect the yoke of the receptacle to the metal box and the yoke of the receptacle is connected to the ground pin whole in the receptacle. If you have an equipment ground run to the box and bonded to it then this type of receptacle would properly ground an appliance.
The ground wire in an electrical system is very important. It gives another path to ground in case of a short, this aids in the trippping of the protection device. It also drains of transient voltages and/or currents that can be induced or formed from static charges. These types of charges can be detrimental to electronic devices such as computers and stereo equipment, etc. In laymens terms it helps to provide a clean electric current. It is very important to have a good ground. If you are not sure how to replace your receptacles properly and provide them with a proper ground then call an electrician out to do it correctly. The price he charges will be offset by the piece of mind you will have knowing that the receptacle is properly grounded. Hope this helps.
[This message has been edited by ElectrcBil (edited October 24, 2000).]