I've done mainly self-drive safaris in Kruger & Etosha/Caprivi strip, but also (organised) trips to Botswana, Kenya & Tanzania...
I normally take 2 bodies (recently both 1Ds Mk III), leave the 500mm on one, the 70-200mm or 70-300mm on the other and only occasionally stop to put on either the 17-40mm or a 1.4x converter on the 500mm. For the "up-close", the 70-200mm "wins", but I would say a high percentage are shot with the 500mm lens and I just move the vehicle to where I want to be...
Most recent trip I played with a 7D just for video, which was cool with the 500mm lens

Are you doing an organised trip? If you are, then make sure you understand the vehicle(s) so you can make appropriate decisions on support for any of your lenses.
I can't think of many times I've yearned for a 400mm or 300mm over using the 500mm. In fact many times, it's the opposite, I'm worrying about DOF @ the subject distance to ensure I can get 2 or more in focus without losing too much speed.
Even on organised trips, especially those specialised photographic trips, then the driver will normally position you based on the glass of the shooters, and most seem to stick with the 500mm or 600mm.
200-400mm would definitely be interesting in the future, however, if money is no issue, then take a 300mm f/2.8 - but probably more as a backup

Few other tips - I bought a little sandisk memory holder for the most recent trip. Has a ring so you can attach it to the camera strap, which was easier. A pair of hyperdrive ipads were there for backups, and the ipad for preview (alas did not have a wifi x2 at the time). Oh and bought a coffee mug which plugged into the car cig lighter and boiled me coffee for breakfast in the morning...
Happy to provide any other info which may help, especially if you are doing self-drive....