If all you do with the graphics card is 3d works, no gaming than any cheap cuda enabled card should be more than powerful enough. honestly looking at your system it looks like you do not really need such, but ok.
opengl 3.0 is newer, it is a free alternative to direct X. (correct me if im wrong) windows games and interface does not use it, only freeware or specific programs. so nothing for you to worry about. if you get a new card it will have 3.0 most likely anyways.
there probably is an octacore supporting mobo somewhere, but it will not be cheap and probably only available for servers.
your power supply is more than powerful enough to run today's most powerful graphics cards.
I believe any 9 series and above from nvidia supports CUDA, but make sure you check before buying. 9800GT and above definetely supports CUDA.
good enough for you???