Im guessing that because DX9 games are not designed to take any particular advantage of DX10 you will see no appreciable change in the way the game looks.
Im running an ATI Radeon 8800 GTS card at the moment, Flight Simulator X rocks with the DX10 settings for example... however i cannot notice any notable performance boost or graphics boost on any of my DX9 games like SH3.
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