DirectX10 is Vista only.
DirectX 9c is the latest XP one. It's one of the most important things that would affect performance, so definitely install the latest version.
DirectX10 wouldn't offer much improvement to Spore -- it's gain is more with realistic stuff.
Just because Spore isn't DirectX10 designed, it should work fine with v 9c which is the latest XP DirectX.
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