Depends on how you define 'need'. If you're happy with 8.1 so am I. I use 9.0 with win98 and winXP just because it's available (and directx8.0 with win95b). Not being a serious gamer I can only say that I've never noticed any improvement with 9.0, but then I don't think I've ever seen much improvement after installing any of the stuff I've download from Microsoft...except perhaps for the shutdown supplement for win98, I think that one helped.
Most games/apps that require a certain DirectX version will have it on the cd, so you useually don't need to download it. I only know of 1 game so far that requires it (freelancer.) I'm sure there's probably more already and in the near future many will require it. However I would say don't bother unless you need it already. I downloaded it for programming purposes, but I still do most of my audio/video programmin with DirectX 7.
I'm not sure about the OpenGL, I never use it. I believe it depends on your videocard driver and the app you're using.
I don't think that file contains all the necessary stuff; it might even be just a software OpenGL implementation. But all the stuff you need for it will be installed along with your video card drivers.
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