I'll reiterate a couple important points already made before offering my conclusion:
- We don't know what a D400 is, so there's no way to speculate its performance against a D800 in DX mode. - Buying a D800 only makes sense with the intention of using it in FX mode. Crop performance is equivalent to D7000 which is available sub-$1000, less than 1/3 the price of D800. It makes no sense to spend $3K for that level of performance.
To the question - assuming the D400 is a DX camera, there's absolutely no way a D800 in DX mode should in any way compete with it (and if it does, Nikon shouldn't bother with D400 DX). The D300s offers high pixel density and high frame rate, compared to its FX counterparts. The D400 should do the same compared to D800. If not then don't bother with it. 24MP DX 8fps in a D800 body.