FS9 was written to use DX9, with Windows 7 FS9 will run just fine with addons. So will Vista DX10 is just a program language to display the 3D objects on your screen along with your graphics card. It will work with DX9 programs and under as the DX9 language is still there it is just improved. But not in the sense of performance as FS9 isint coded to use the new feutures of DX10 and up