If you have an ATi card, go and install these drivers > Catalyst-DOOM3 Hotfix. The Direct link is here. Note: These are BETA HOTFIX drivers are not suported by ATi in anyway and they are only for people looking to get a bit of an extra perfomance boost when playing Doom3.

Nvidia folk, just ignore this step, good idea to have your latest drivers installed though.

Install DirectX 9.0c, this is a new one and it helps quite a lot. It can be found here.

Defragment your harddrive, you might think this might'nt help much, but it does and it gives a large hand in reducing stuttering in games. Not to mention load times. This one alone, brought me up about 10 fps.

Set your card settings to 'Optimal Performance' rather than quality. For users with an ATi card just right-click your desktop > hit properties > select the settings tab > hit advanced > select 3D and move the slider. For Nvidia users its probably in a similar place (destkop > properties > settings > advanced).

Note: You should have a [un]zip client that can unzip files into their set directories, WindowsXP comes built-in with one. If you don't have one, you can download WinZIP here.

The pk4 files (pak000 to pak004) Doom 3 installs in the 'base' directory of your Doom3 Installation directory ('C:\Program Files\Doom 3\Base' for most people), are actually just zip files. Doom ussualy extracts the relavant 'pak' when the associated level is being loaded, and then deletes the extracted files when you quit the game. So, if you do it now, once, Doom uses the extracted files and hence greatly reduces load time.

-Rename the files pak000.pk4 to pak004.pk4 in your 'base' folder of your doom installation directory to pak000.zip, pak001.zip etc.
-Then extract the files into the SAME directory.
-Finally, delete the pak files.

If you did it properly, Doom3 should now use your newly extracted files to load your game and hence load much quicker. Don't forget the last step, or Doom will continue to extract and delete the paks like before instead of using your new files.

Doom 3 doesn't do a very good job of detecting your settings properly, your graphics card ram etc. So Theres some things you might want to change in the config. NOTE: Before you do any of the following, make a backup copy of your DoomConfig.cfg file (its located in your 'base' directory like before).

Now theres 2 ways to change the settings, you can either make a new file called 'Autoexec.cfg' which Doom3 will execute everytime you run the game, or you can edit the file DoomConfig.cfg settings file and change it in there. For the purposes of this guide im going to assume you are making the Autoexec.cfg file, to edit the DoomConfig.cfg isn't much different, the setting names/properties are the same, just find the current settings/values in your DoomConfig.cfg and change them.

-Make a new empty file, in for example notepad, and name it autoexec.cfg

First, to show your fps during the game in the top corner all the time, add this line:

set com_showFPS "1"

To set how much ram your video card has manually, use this line: (Where 256 is the ram on your videocard)

set com_videoRam "256"
You'll probably have some of the following settings out as well if you had the above set wrong by doom.
In the following, 1024 is for 256mb cards, 512 is for 128mb cards and 256 is for 64mb cards. The second line ussualy just enables the setting above it.

set image_downSizeBumpLimit "1024"
set image_downSizeBump "1"

set image_downSizeLimit "1024"
set image_downSize "1"

Check your current settings before change the next one. In it, cacheMink is related to cacheMegs, and you should increase the other if you change one. You should set it between 96 and 128 depending on your specs.

set image_useCache "1"
set image_cacheMegs "128"
set image_cacheMinK "10240"

You might also want to set the Monitor Refresh Rate:

set r_displayRefresh "75"

And now for some codes which change the game:

set g_fov "90" -Increases Field of View
set r_brightness 1 -Increases Brightness
set r_gamma 1.3 -Increases Brightness
set g_nightmare "1" -Enables "Nightmare" mode (the hardest singeplayer difficulty)
set com_showMemoryUsage "1" -Shows memory usage during the game (like showing the fps)