Whether you get a Nikon D80, Canon 30D or Canon XTi/400D, that does not matter much... it is the lenses that you need for what you exactly mean with "nature photography" that will make the difference.
So... what do you mean with that? You want to bring far away animals closer? You want to make landscape photos? And what would be your budget?
Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30