What gives you that idea? First gen D3D10 cards (G80/R600) should handily spank any current cards out in DX9.
I dont know, dont forget that in vista DX9 games are emulated so in when running DX9 games the DX10 cards will possibly end up having lower performance then the DX9 cards due to the absense of DX9 backwards compat in vista. Second generation DX10 hardware is were it gets exciteing