Ollie,
Before you ask, the D70 uses the EN-EL3 battery, the same as your D50. The D70s uses the EN-EL3a battery which is compatible with the EN-EL3 battery.
Personally, I think it's better to invest in better lenses rather than fancier bodies. Unless you are regularly finding that there's things you simply cannot do with your existing body, I'd advise on better lenses rather than a new body.
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