Going on Safari - but which camera XS-1 or Nikon D700?

Then bring your D80, and D700, and use them situationally.  You won't know how far away subjects will be until you get there.  You could research the exact area you are going, perhaps talk to other photographers online who have taken the same safari that you'll be doing.

The the X-S1 is not a bad idea as it will get you to 720mm.  The Canon XS50 goes to 1200mm and has very good IQ for it's sensor size (including RAW like the X-S1).  I just don't know if you'll need that kind of reach.  The animals on safari are quite large, in general.  It's not like shooting small birds where you do absolutely need over 600mm.

Gearing Up for Safari DPR Article

http://www.dpreview.com/articles/8065882473/gearing-up-for-an-african-safari

If I were you, I'd rent a long lens, perhaps even a 300 f4 which works quite well with the 1.4 and 1.7 Nikon TCs.

A small telezoom like the Nikon 70-300 VR is useful, but it doesn't work well with TCs.

The best lenses for TC are the 300 2.8s and 400 2.8.  Renting one would be far less than the price of an X-S1, but they may be more weight than you want to carry.  That is why I suggested looking into a AF-S 300 f4.

By the way, there are tons of articles online about planning for safaris for photographers.  Good luck, Markus