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"They will see the effects OS X does now, on cheap video cards, and they will see the effects that Linux does (assuming this goes anywhere) with the same class of hardware, and then they will see that Longhorn does the same effects, but requires a DX 9 level card."
You are making a lot of assumptions:
1) OS X, Linux, and Longhorn will all implement the exact same UI eye-candy features.
2) Using the DX9 feature set will not be inherently faster than using the feature set that OSX and Linux use.
Mid-range cards support DX9 currently. Its not a matter of power -- it quite possibly is a matter of features for what they intend to do.