I can't say much for Botswana but I can help a bit with Namibia as I just got back two weeks ago.

I was only in Namibia for 4 days. I ended up renting a 4x4 Hilux with Hertz since the other local companies (the ones that rent camping packages as well) were unwilling to rent for such a short period of time. I spent two days down in Sossusvlei and two days driving (one down and one up) to/from Windhoek and had an awesome time.

You'll want to check with the rental company whether you can drive across the border or not. One thing to keep in mind though is flying WDH-VFA-SA. Flights were cheap when I looked, especially considering the fuel cost of driving there and back. Diesel runs around $3.80/gallon or more. Maybe consider renting the 4x4 only for Namibia, fly to Victoria Falls then onward to South Africa from there?

Depending on your plans in South Africa, Chobe may or may not be worth the time. If you plan on hitting Kruger when in SA, I'd say skip Chobe.

If you want to read about my trip, which was more SA centric than Namibia, it can be found here...

http://www.flyertalk.com/forum/trip-...ei-kruger.html

Just PM me if you have any further questions!