ADSL packages available can vary quite significantly depending on your location and the type of ADSL infrastructure that is in place in your area. Cheap ADSL is dependent on the lines being available and the capacity of those lines, so check if your ADSL is cheap and nasty or cheap with acceptable speed.
Cheap ADSL packages are not all equal and you need to check if it is capped or shaped before entering into an agreement.
ADSL packages are becoming cheaper and cheaper and offering now include unlimted downloads with some shaping or uncapped unshaped ADSL.
Cheap ADSL is generally available from Telkom or wireless broadband from a number of wireless broaband providers like Neotel.
Unshaped, uncapped ADSL packages should give the maximum advertised download speed all day.