Depends on how you want to run the game. Are you happy with 800x600 @ low / medium settings @ 20-30 fps ? Then your 6600GT will suffice. Do you need more FPS ? Higher graphics settings ? Get a more powerful card.
Let's see how powererfull the X3850 and X3870 will be. (DirectX 10.1 and ShaderModel 4.1)
And Nvidias 8800 GT are good too. (Only DirectX 10.0 and SM 4.0)
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