I've read about several boosts. The one your described I did... however got little difference. I usually got 38 FPS on Doom 3 on medium details and 800 by 600 (tested it with fraps) i tested it with the 32 changed to 96. only 40 FPS. I have 1.5 gigs of ram. I changed it to 328 and got a bigger shift it stayed solid 45 fps.. future changes did nothing.

There's another performance boost (don't do with the demo)
Go into the base Folder in Doom3 Copy the pak4 files somewhere so you have them and can revert if needed to join an online game.

Pak000
Pak001
Pak002
Pak003
Pak004

Take each pak file and open them with WinRar
When you open them using winrar, they will appear as folders, not zip files, take each file and left click and drag the folder into the base folder

pak000pk4 has a LOT of files, select all and do the same thing you did with the others, when prompted to overwrite, click yes to all

When you have completed this process you will have a bunch of folders (maps Textures, Sounds, ect) in your base folder.

Delete or move the zipped PK files that you just unrar'ed

When you play now you will see an insane performance boost!