tl;dr: Go here and download the appropriate patch installer for your game.
NewDark is a new game executable for Dark engine games which appeared out of nowhere in late 2012. It was created anonymously, but has been extensively tested and found to be completely safe, to the point that GOG includes it in their System Shock 2 release.
What NewDark does is upgrade the Dark engine to support many new and incredibly great features, such as full DX9 support, high resolution textures, native high resolution, widescreen and multi-monitor support (including extremely high resolutions like triple 2560x1600). All of these features work in Thief 1/Gold, Thief 2 and System Shock 2.
Due to the odd legal nature of the patch (not blessed by the original companies, etc.) I will not be hosting it here at theifmissions for the forseeable future, or even link to it directly. You can find info on where to get it here.