I can't say that OC'ing stuff leads to problems, seeing how I am OC'ing my x800 Pro from (475 / 450) to (542 / 470) without any problems.
But if Carmack himself said it.. then give it a try.
Here is a guide to tweak Doom:
http://ucguides.savagehelp.com/Doom3/FPSVisuals.htm
Generally, people claim these settings help the most:
Quote:
image_useCache
image_cacheMegs
image_cacheMinK Controls memory set aside for temporary loading of full-sized precompressed images and files.
WARNING - When set incorrectly this can cause Doom3 to crash at startup or cause instability/graphical artifacts in game.
Set cache megs to a higher value if you have 512 megabytes or more of system RAM. Guideline is 1/6 to 1/4 of your system RAM.
Set image_cacheMinK to 2048, this can be lower but may cause Doom3 to crash.
Set image_useCache to 1 to enable background load image caching
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There is also a setting where you configure the amount of RAM your video card has... I belive its:
where X is the amount of memory your video card has (in this case 128).
Check the guide if you need help on how to implement those.
Be sure to back up your DoomConfig.cfg before doing this because if you change the wrong thing it will cause the game to crash even more.