Nicholas, hi, yes the Argus (in Cape Town) is the largest single day cycling event in the world, with, as Vo2 has said, more than 40 000 people entering. I think about 3 000 foreign cyclists. Check out the official site at www.cycletour.org.za. Plan a trip to South Africa and do the race, it is a wonderful event. Besides, we can do with every Aus dollar you can bring. There is still time to enter, it can be done online.
The other event is the 94.7, where they expect about 20 000 entries this year. This race is in Johannesburg.
I also think this race is tougher than the Argus.