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You will get better dynamic range with a D700 over a D90 but that is more to improvements in sensor design than sensor size. If you compare a D300s to a D700 you will only notice a diference over 1000 ISO.

There is a lot of hype over the high ISO of the D300s and D300. Resolution and noise is low, but colour goes flat it high ISOs. I use a Fuji S5 (DX) as well as Nikon,  this has 6mp + 6m smaller cellls for highlights. I recon the Fuji has better DR amd colours than the D700 at 1600ISO.