audiomode: 1 is stereo-, 2 surround-sound.
audioquality: 1 sounded better than 0 to me, but that might be imagination.
brightness: Ranks from 0 to 100, sets the brightness in full screen. You can set higher values, but that just looks ridiculous.
basetexturefilter: 0 is no, 1 a bad and 2 the best texture filter.
basetexturelodbias: If you set it to a higher value than 0, distant objects will be loaded with lower details to save power. But sometimes it begins with cars five meters away from you. Not yet sure about the possible values.
basetexturemaxani: Sets the Anisotropic Filtering (AF) for any texture. You may set 0x, 2x, 4x, 8x or 16x AF.
carenvironmentmapenable: Reflections on cars are controlled with this. Ranks from 0 to 4.
carlodlevel: 0 for minimum and 1 for maximum details on cars.
enableaero: 0 to disable Aero and 1 to let Aero on while playing. Just important for Windows Vista and 7.
firsttime: 1 for a performance check of your system and a automatic graphics-preset based on it. You can enable it with the "Default"-setting in-game. It is stored in the UserSettings.xml at <VideoDefault>. Let it at 0 unless you have installed new hardware and want a new default-setting.
forcesm1x: "True" just causes graphics errors, maybe it is for compatibility to older GPUs?
fsaalevel: Full-Scene Anti-Aliasing, renders the 2D-image again through the Anti-Aliasing-process. You can set it from 0 to 4. If you prefer the AA of your graphics driver, set it to 0.
globaldetaillevel: Sets the details of the environment, ranks from 0 to 4.
maxskidmarks: The higher this setting, the more skidmarks you can have at once. With a higher value they won't disappear as fast. Ranks from 0 to 2.
motionblurenable: 0 disables and 1 enables the Motion Blur-effect.
overbrightenable: 0 disables and 1 enables the Overbright-effect.
particlesystemenable: 0 disables and 1 enables the particlesystem, which is responsible for powerup-, smoke- and fire-effects. If your computer lags while using powerups, you may set it to 0.
performancelevel: This controls which of the five presets in-game is loaded. Ranks from 0 to 4, but as you don't want to have a preset if you do this, let it at 5. Then there is no preset to be loaded.
pixelaspectratiooverride: If the image on your screen is too big or too small, you can adjust it with this setting. Just enter the number you want to multiply the size of the image with.
rainenable: If there was a rain-effect in World: 0 disables and 1 enables the rain-effect. But there is none yet.
roadreflectionenable: Controls reflections on streets. Ranks from 0 to 2.
roadtexturefilter: Look at basetexturefilter, for roads.
roadtexturelodbias: Lood at basetexturelodbias, for roads.
roadtexturemaxani: Look at basetexturemaxani, for roads.
screenheight: The height of your monitor in pixels, e.g. 1080 for Full-HD.
screenleft: I just don't know what this does, altering it changed nothing for me.
screenrefresh: The refresh rate of your monitor. Should be set automatically to the right value.
screentop: I just don't know what this does, altering it changed nothing for me.
screenwidth: The width of your monitor in pixels, e.g. 1920 for Full-HD.
screenwindowed: 0 disables and 1 enables the window mode.
shaderdetail: Controls the quality of the shader, responsible for changes on surfaces, e.g. water, scratches on cars and so on. 0, 1, 2 and 4 are possible, 3 just causes errors.
shadowdetail: Controls the quality of the shadows, it is recommended at 2 with shaderdetail at 4 for best shadows. Ranks from 0 to 2.
size: Might control how much VRAM World can use. In MB. 0 is autodetect. A manual setting saved FPS for some people, you may try it.
version: I don't know which version is meant, it hasn't changed for the last weeks.
visualtreatment: 0 disables and 1 enables visual treatment. Responsible for effects while crashing, entering or leaving the safehouse or warmer/colder colours at specific daytimes and places.
vsyncon: 0 disables and 1 enables VSync.
watersimenable: 0 disables and 1 enables water simulations. In World they are really bad, you might set it to 0 as there isn't much water in World.