Hi Andrew,
Sounds like you have a great trip planned.
I know Pilansberg very well; presumably you'll be staying at Sun City or one of the lodges there (Bakubung for example). You can either drive yourself around the game park, or there are companies there at varying cost that do drives. But Pilansberg is a volcanic crater, circular, and the game park is artifically stocked. It's not too large either, so a day max, maybe two absolute tops. Disappointed that the bird life is restricted (minimal).
Kruger Park, not that far away, is a much better bet.
Namibia, now there's a wonderful country, are you going to Etosha, the skeleton coast, Ricktersfelt? Swakupmund on the coast is worth a visit too.
Mike