some of the latest games will look for Dx verson and install Dx 7 or 7a if it is not found anyway. I've been using Dx 8, (build 146), for a while now with out issue and the final build should be soon as ME is out now.
Only going on my experiences here, but I would say that d3d is good but not as good as what opengl can do.
But I have heard that dx8 has quite a few more tricks for d3d.
Just have to see whether games make use of these extra features, looking forward to texture compression though, cos it does wonders for the frame rates in opengl.