Originally Posted by tweakguides.com
This file doesn't exist by default, so you need to create it by going to your \Program Files\Electronic Arts\Crytek\Crysis directory, right-clicking on an empty spot and selecting 'New>Text Document'. Then rename this .txt file to System.cfg (not System.cfg.txt). By default Crysis looks for the presence of this file each time it starts, and if the file exists and contains valid commands, they will be automatically executed when the game loads up. As such, this file is the perfect place to store most of the commands you wish to apply to Crysis at startup.
The format for entering a command in a config file is different to that of commands used in the console. If you want to disable motion blur for example, the console command is r_motionblur 0, but when entered in the config file, it needs to be r_motionblur=0, and every command needs to be on a separate line.
Note further that some people recommend creating an autoexec.cfg file in the same directory, instead of, or in addition to System.cfg. This is fine, since any commands in such a file will also be executed automatically when Crysis loads up. However it appears that any commands in an autoexec.cfg file are applied after the game has loaded, and not necessarily during startup, and this may impact on whether certain commands are successful or not, so on balance I recommend using System.cfg. The help text for several of the console commands also refers to System.cfg, so it's best to use this file (e.g. see the help text for r_Driver).
Importantly: Some system commands still cannot be altered by using System.cfg, autoexec.cfg or any other custom user .cfg files. It appears they can only be altered when entered in an original Crysis system file, such as one one of the difficulty .cfg files (e.g. diff_normal.cfg). These commands are denoted by (system) on the following pages, and you should see the Diff_ file section further below for more details.
Finally, you can always execute commands in any existing or custom .cfg file you place in the same directory as Game.cfg by using the exec command. E.g. create a Mytweaks.cfg file, fill it with various commands, and then within Crysis, at any time you can open the console, type exec Mytweaks.cfg and they will all be executed straight away - though clearly commands which need to be altered at startup won't work this way.