My friend tried this trick, he said he didn't notice anything really drastic.

BUT, he did come across this that might be helpful to some:

On the doomconfig file " image_cacheMegs "256" "originaly it was 32...
He changed it to 256 and saw 10 frames increase.

Normally at 1024/768 his average frame rate is about 21.6 and highest would be 60.

But since he did the change, the average frame rate is 31.6.

The file details: doom3/base & name of the file is Doomconfig.cfg

This might give pretty good results for some. He mentioned that since he's only running 512Ram, he tried to change it to 512, but didn't really help, he anticipates getting 1g of Ram and is confident that results will be even better.

Hope this helps!