OpenGL? Hmmm,..that would be a big mistake.
Consider this, any opnegl based anything is at the mercy of each video card maker. At least with DX we all have a common API and can work around known issues with it.
With opengl,..you then have code for nvidia, ati, and so on.
There is no real good reason to go opengl. Opengl actually has missing features that DX8 supports. Yes, the OpenGL fan club is addressing these by adding new extensions to OpenGL, but which video card maker is going to go back and rewrite all thier opengl drivers for the cards that have shipped the last 3 years?
OpenGL is just another graphics API, just like DX is. At least DX is a common denominator. With DX you get a pretty consistent API that forces the video card guys to write consistent interfaces for. You do not get that with OpenGL.