Panny DMC-GH2 or What Else?

I need to be able to get up to 400mm zoom (that's after crop factor calculations) for a trip to Africa in the fall. This is my most limiting issue (along with size/weight for travel/trekking). I am a beginner that wants to really learn decent photography and take nice shots to hang on my wall.

The Pan DMC-GH2 will get me to 400mm via two lenses: Pan G Vario 45-200mm (some say it's not very sharp) and Pan G Vario 100-300mm (wow-that's a lot of zoom). (And I think I now need to understand the teleconverter add-on possibility for such a long zoom, right?)

As alternates, I was looking at the Sony NEX-7 but there's no lens for this much zoom. Also, looked at Nikon D7000 with a 55-300mm, 28-300mm, 70-300mm, or Sigma 18-250mm lens to get me enough zoom for the safari. But then how big/heavy would this beast be for travel?

Any thoughts here? I'm down to zoom reach, size and weight as big concerns...oh, and nice photos.