while the %'s have shifted since that was last updated, I'm sure Vista/Win7 have not taken over in a combined % over XP and if you refuse to create the DirectX 9 rendering .dll (read above) you are in essence cutting out at least 1/2 of your potential buyer base which is crazy. You'll have to learn DirectX 9 anyways to support them, so might as well start there.
According to the latest Steam hardware survey, users with both Vista/Win7 and DirectX 10/11 GPUs are now on 56%. Total Vista/Win7 is 71%/72%. XP is down to 20%. DX11 looks more like the reasonable choice every day.