first off, u probably have it set to medium quality but to no avail, bc i doubt u would even be able to load a level with ur computer, not saying mines much better... but for one thing, no matter how shity ur graphics card is, 256mb video ram will seriously help on doom3.
the game.. if ur using under system requirements like me, assumes u have a 64mb videocard, well mines 256
in console i cahnged alot of shit that sepht was using, all set for 256mb video, when i first installed doom3 it was unplayable lag, i got thru the first "level" before any invasion or anything and when i got to loading the second level it was a black screen for 40 minutes.
i changed my config and now im up to the invisible girl... btw what the hell is the deal with her... anyway i get anywhere from 15 fps killing a bunch of shit to 30 fps in a big area to 60+ fps just walking down halls and shit, i hiccups when i open doors and when a demon first spawns in a room but thats about it...
my specs are
geforcefx5200 256mb 250/400 overclocked to 300/500
pentium 4 1.3 ghz
128 mb rdram
one of the things i changed was cachemink to 10240 that helped a ton as i have alot of video ram to waste, and not nearly enough system ram to use..


... this far into the post i realise that since i know what im doing, although i use a shity pc, i can optimize things to make it work unlike u... ur screwed, next time u buy a game read the min requirements. im getting a amd 64 3000+ neo motherboard 512 ram and tt hsf for the cpu, which ill be overclocking , ill be sure to tell u how badass this game is .. hehehe

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I will only complicate you. Trust in me, and fall as well.
amd64 3000@3300 , R9800PRO128@XT