Every game is different. Some games support multiple API's, some don't. Half-Life and its variations (CS) work best in OpenGL since it is based (loosly) on the Quake engine). UT can run in Glide, D3D, MeTal, OpenGL, software renderer. You can pick the one you want. On your card, D3D is the best API for UT unless you use the Loki OpenGL "special" driver.