There is no mention of DX8 on Microsoft's web site and I doubt there will be a official download before Windows ME is in retail format.
I've been using this Beta version for a while now with no problems but use at your own risk. It can only be removed properly with a reformat and it could be date limited, you have been warned so I don't want to know if it screws up your system.
I used it and found no difference. No games support it yet so you will see little to no difference in game play. Plus if there is a problem you have to re-install Windows and then install good old DirectX 7.0A. I would just stick with 7 until 8 is acually needed or at least until it comes out of Beta.