If you load the game in DX10 mode, it'll work in DX10 no matter what settings you choose. But there is little reason to load the game in DX10 if you're not using settings above 'High' as DX9 generally out performs DX10. If you're thinking about installing Vista solely for DX10 in Crysis, I wouldn't bother. XP still outperforms Vista in most cases. Not all, but most.
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