The D70 is a better camera than the Rebel 300D. However, the Rebel 350D is a pretty nice camera which would make a provide a more difficult choice.
The only DSLR that really has any substantial weatherproofing are the Olympus DLSRs. However, they are more expensive and lenses cost more. The high end Nikons and Canons also have some weatherproofing, but are several thousand dollars for the body alone.
I personally wouldn't worry about the weatherproofing too much. It is a tool and it is meant to be used. I woudn't go and change lenses outside in a dust storm or on a rainy day in the forest, but otherwise I wouldn't have a problem. As long as you are careful, you should be fine.
Now, if your choices are the D70 and the D300, I would choose the Nikon everytime. If you are considering the new Rebel XT (D350), then I might be more inclined to ponder, but would probably choose the Nikon anyway. It is a much more user friendly, yet powerful camera. The extra 2MP the 350 has are irrelevant to most herp photography. The Nikon has faster flash sync, easier to use controls and is very responsive.
Try reading some review at sites like dpreview.com.
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Chris Harrison