Is it better to go north-south (Jo’berg-Kruger-Durban-Cape Town) or in the opposite direction.

I’m going to be heading for Cape Town then travelling along the south coast a bit. Any places i must not miss, things i should do or places i should stay. Am going to be backpacking it and staying in hostels so any money saving tips would be appreciated to!

I have already been to Cape Town and know that to be superb. Which other places (cities in particular) are worth visiting, and why?

I would like to visit South Africa one day on a tour. I would like to visit Johannesburg, Pretoria, Cape Town and Durban. However, I met a South African couple who said that it is not very safe and sometimes strangers can knock on your door and rob you and threaten you with a gun. Is this true? Is South Africa a dangerous place? Or only certaina areas? Are the police effective or corrupt?
What about those cities I mentioned?