I recently had to reinstall my machine (ultimately due to a combination of fried power supply and inappropriate spontaneous reboots) — but in the end I got a large increase in performance for DOOM3. Combined with the memory cache suggestion Jay turned us on to, I was able to run 800×600 (256mb image cache) on an A7N8X-E Athlon XP 3200+, ATI A9600XT at nearly 60 fps constantly on HIGH detail.

It’s WAY scarier than it was before these tips… partly because my system would “lag” a little before creatures would sometimes come out (texture loads, etc.) — now creatures simply come out without warning — and the animations are so creepy at 60 fps! I definately have to side with fps for environment and feel over low fps for better res.

(These tips are what worked for me on an nForce2 motherboard — if you follow this keep in mind that you will need to start from a fresh reinstall of Windows XP (and that I used SP2 updates — not without its own risks) so “reader beware”! This advice is strictly given AS-IS… I take no responsibility if you nuke your system or don’t know what your doing.)

Here’s how I did it:

1. went into bios, and set APIC Mode to “disabled”. Apparently, there are reported performance problems with APIC enabled. AMD went so far as to recommend disabling it to some mobo reviewers of the nforce2… kinda sketchy if you ask me.
2. clean installed XP.
3. installed XP SP2 from MS update.
4. went to nVidia and installed the latest nForce2 unified drivers.
5. installed the latest ASUS-issued drivers for my ATI A9600XT.
6. increase the imagecache setting in Doom3 cfg to 256MB.
7. set detail to HIGH
8. set vsync to on
9. set 800×600 res.

I had all the same pieces before, the only things I changed were: APIC disabled, WinXP SP2. From what I’ve read it sounds like nForce2 owners need to disable APIC — however the catch is that you have to do this BEFORE you install the OS.