your power supply that comes with your computer can automatically take the correct voltage... so you don't need a transformer or anything like that. however, the physical plug will need an adapter.
south africa uses the old style british plug "the elephant foot". it is similiar to the modern british plug, but the pin sizes are a bit different.
the large ground pin is 8.7mm on the s. african, 7.0mm on modern british. the smaller power pins are 7.0mm on the s. african and 5.0mm on the modern british.
you can get one of those "all-in-one" world traveller power adapters (which are usually crap, because they try to accomodate all of the different pin sizes and are generally too loose for convenient use, but might actually be right for the s. african size) or just pick up a $2 power adapter once you get in country.