The first DX9 games will come out with DX8 support.
DX9 will add some extras to the graphics and maybe some games will run more smoothly in DX9 mode.
Don't forget : company wants to sell lot of games. If they only support DX9, they will leave a lot of gamer disappointed.
Of course, if you have TOO MUCH money to spend, buy a mid/high-range DX9 card. On the long run, DX9 is better... But money wise, I think a good DX8 card is still a smart choice.
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Would you buy a GPS enabled soap bar?