Get a 500mm f/4 IS or 600mm f/4 IS first gen.
I love my 500mm to bits, and I'm not sorry for skipping the weight of the 600mm. Although I think I will still get one because I would still find it very useful.
But if you are even slightly concerned with weight, get the 500mm. And you can find them for reasonable prices, I picked up mine at Kenmore camera for $2750 including tax, it looks 100% new with no marks or blemishes. And it is an excellent lens even today. While still a chunk of a lens, significantly lighter and smaller than the 600 which is a monster!