If any of you are still unaware about this trick...well, you're about to be aware. Got this from d3files.com
In your DOOM 3 directory, find "DoomConfig.cfg" (x:\Program Files\Doom 3\base).
Open DoomConfig.cfg in Notepad.
Find the lines:
seta image_cacheMegs "32" (May be "20" on some computers)
seta image_cacheMinK "(Not sure which number thats supposed to be here"
seta image_useCache "0"
anyway, change the settings to the following:
seta image_cacheMegs "xxx" (xxx - Half the number of you RAM, so if you have 512 mb RAM, you should put 256 here, 1024 - 512 etc..)
seta image_cacheMinK "20480"
seta image_useCache "1"
seta image_useCache "1" - This enables the memory cache tweak, if this is set to 0, it would be disabled, logically..
seta image_cacheMink "20480" - Im not 100% sure what this does, i think it has something to do with the minimum use of cache in Kilobytes... Anyway, i heard this is the number it should be on..
seta image_cacheMegs "xxx" - The amount of ram the program should use (In this case, Doom 3). I advise you only to set this number to the half of the total RAM you have, or it may take alot of CPU power and that will result in the occational 3 - 5 second lag when you open doors and stuff.