Very few of the international payment gateways support South Africa because of the strict banking laws. You will need to look at one of the local gateways and write (or get someone to write for you) a module for the cart if one does not exist.
Gateways might look at:
http://www.setcom.co.za
http://www.payfast.co.za (They have a gateway module for WP eCommerce)
http://www.mygate.co.za
http://www.netcash.co.za
Good luck
