Seems the api of DirectX 10 will not be backward compatible with DirectX 9.0c.. Now I don't know when DD2 will be released but if it's after or near the launch of Windows Vista maybe it’s worth wild make it DirectX 10 compatible since DirectX 9.0c is only supported by emulation witch means that there will be a performance loss. Then again I don't know if DirectX 10 will be available for XP.
Hard design choice to make
softpedia 
