When starting the game for the first time it will take you through the setup process. When the video tab is clicked it will start the video device selection. Select OpenGL, deselect Direct3D and 3Dfx Glide, and then click the Scan for the Selected Video Devices. This selection will allow you to set all sorts of settings with you video cards control panel and be reflected in the game.
Next Click on the Speed tab. Click on the drop down tab and select Very High. The auto-detect doesn't read modern hardware properly.
The last but not least make sure to update your network settings on the Network tab.
When you are done click OK and then you can launch the game with the Play button.
The warning about OpenGL stems back from when Descent 3 first launched. At that time OpenGL drivers were a bit dodgy. Since then all the major manufactures have gotten on board with OpenGL.