The horrible advice first:
Try to split your backup gear into a separate bag, one that you could survive with(photographically) if all your main gear was taken. Include a backup wallet with a credit card, etc. Keep that backup bag locked up in a safe place in the hotel/lodge but make sure it stays in a physically separate location to you. This applies to checked luggage too.
Make sure your insurance is up to date.
Don't be tempted to fight if someone does try to take your stuff, it's just stuff after all and there's no point in photos if you're dead.
For street stuff - take the advice of a local guide. If you are on a tour of Soweto for example you will probably be fine even with really expensive gear. There are other places where you would be lucky to escape with your life regardless of whether you were carrying anything of value at all.
Check out these guys for an awesome day if you are able to spare one in Johannesburg:http://www.soweto.co.za/html/t_soweto.htm
A sturdy camera bag with Pentax or Tamrac or similar emblazoned on the side is the same as a big sign saying MUG ME. One technique I have often used in place that were pretty dangerous is to simply bring along a plastic bag that is black and completely opaque and quite strong. The kind of thing you often get from book stores. It will be strong enough to carry a decent sized SLR with lens and you will not look like you are carrying anything expensive in it. Doubling/tripling-up with simple supermarket plastic bags (use local ones) will work well and will work well even with fairly heavy cameras. Then you take out your camera, get the shot and put it back in the bag.
If you are inside a lodge/reserve then you will most likely be safe.
If you are hiring a car, be careful on the roads until you adjust to the local driving style. I like to think of them not as public highways, but rather one giant race track where Ferraris race with Porches, race with long-haul transports, race with taxis, race with clapped out 1957 Nissans. Everyone goes as fast as they can. Takes a while to get used to and you are probably more in danger from a car accident than from criminals.
If you are going into malaria areas then make sure you take the tablets - and you usually have to start some days/weeks before your trip. They can be nasty, but not nearly as nasty as malaria. If you are in a malaria area and feel like you are coming down with the flu seek medical advice immediately. Take lots of deet with you.
Finally, don't get too freaked out by the danger side. Take all sensible precautions then just try to forget about it and enjoy yourself.
Now for the good stuff:
South Africa is beautiful. Quite simply breathtaking in so many ways, with amazing photo opportunities everywhere.
If the DA* isn't out then the Sigma 100-300 f4 is a very high class lens for your telephoto choice. The Sigma 80-400 isn't too bad either, and of course there is the 50-500 Bigma.
The "Golden Hour" is more like 20 minutes in the north of the country, but those 20 minutes can be amazing. You are very close to the tropics and the sun sets quickly. So getting the right light can be difficult. Many of the best local photographers work WITH the harsh midday light though instead of avoiding it. A levels adjustment in post will often change a picture that looks very washed out into a super-saturated shot that has lots of opportunities. And an adjustment the other way - i.e. making things even more faded and washed out can be very interesting and evocative too.
Think about and practice how you will deal with extremely high-contrast sun/shade situations. Because you won't always be able to be there when the light is conducive to perfect landscape photography. Some of those shots will convert naturally into interesting monochrome shots.
Plan to do lots of chimping until you get used to the light. On the plus side, there is so much light that fast lenses are often not needed.
Take lots of storage with you, there are a lot of pictures out there.