Hi my info is probably rather out of date as I did this tour in 2011. However, I brought US dollars for the Zambia add-on as we had to pay in cash. Given the security situation in SA, I'd be a bit wary about going to a bank there - certainly don't go alone (this is the advice we were given and it includes going to an ATM). You may be able to pay for the helicopter trip/microlight by credit card - it would be worth checking this out. Otherwise you will most likely have to pay in dollars.
Perhaps someone who's been more recently will be able to help better!
This is a fantastic trip, still my favourite and the Vic Falls add-on is the icing on the cake!
Enjoy it all,
Bob