First of all - never get into such a situation. Simply don't drink and drive! Word!
The limit - Your blood may not have an alcohol content of more than 0.05%!
Consequences - If you are stopped at by the police, and if it is determined that you have consumed more than the legal limit of alcohol (remember its 0.05% only!) that you may legally consume while driving, you will be arrested and charged with Driving Under the Influence of Liquor. You will be held in custody until you are able to post bail, you'll face a minimum fine of R2 000 (or a two-year prison sentence, or both). You may also lose your driver's licence, or have it suspended. And, of course, you will have a criminal record!
A word to the fellow tourists in South Africa - if you are going out for dinner, enjoying the Cape Winelands or simply going out partying, pls take a taxi or book an organised tour (winelands). It is not worth it to drive drunk when a taxi costs you around R10/km. Remember - you are on a holiday and enjoying your time! Or do you want to end up with the police after a stunning dinner at one of SA's famous restaurants?