What's the difference between DirectX 10 and DirectX 9?
In Civilization V, when starting the game, it offers me the option of using DirectX 10 or DirectX 9. Are there any graphics quality or performance gains present when using the DirectX 10 version? I know that it would let me turn up the settings higher (for instance, i could enable anti-aliasing in the DirectX 10 mode and couldn't in the DirectX 9 mode)
The most critical part of your choice is "Does your graphics card support directx-10 (or 11)?"
If your graphics card does support higher versions, its usually best to run with the most recent version, to get whatever bugfixes, feature improvements (advanced lighting I believe) and performance gains.
Naturally, if your card does not support the higher versions, you should NOT try running the game in that mode, and stick to Directx9 instead. As StrixVaria says, running the game in that mode should simply not work, (that is what not supported means after all) but the best you can hope for if you do try is broken graphics or the game crashing.
Actually, I don't even think DX10 is backwards compatible. If your video card doesn't support it, you can't run it.
@StrixCaria I was trying to give that impression with my last sentence, but I don't think I was very clear. Let me see if I can improve it.