CS is not an \"openGL\", it\'s a 3D engine using OpenGL, so it\'s part of the game, and you can\'t change it. I think Crystal Space can also use Direct3D but it wouldn\'t be any faster.
But CS is supposed to improve (new release in a few months I think), and remember that even if it\'s not as good as most recent commercial games, Planeshift\'s graphic engine is the best you can find for a free MMORPG.