Excellent suggestions above, it has been many years since my wife and I went to Tanzania and Rwanda, before I started using dSLRs. Like your planned trip, our time on safari was in a private Land Rover, just my wife and I and our driver (the safari portion of our trip was about a week, we also spent a week on Zanzibar for diving and a week in Rwanda as well, with a wonderful trip to see mountain gorillas). I brought a 4 MP superzoom P&S, FF equivalent of 38-380mm.
I won't recapitulate the excellent advice above, but in the large NPs mentioned by Grant I found the longer end of the focal range to be important for most of the wildlife in the larger parks like Serengeti.
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However, if you venture into the Ngorongoro Crater you may find that the animals are quite comfortable around tourists and approach very closely.
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Enjoy your trip!
