The Official DOOM 3 [H]ardware Guide on [H]ardOCP is online, just in time for my own speculation on the topic. Here is a bit from the intro that lays out the goals of this project:

id Software and HardOCP have teamed up to help answer your hardware questions. We're publishing this guide before DOOM 3 goes for sale, so you will know beforehand what to expect in terms of gameplay immersion by your current hardware. We also want to help maximize the overall DOOM 3 experience for those of you that have been waiting for the opportunuity to upgrade.

This guide is not an attempt to crown a king in the realm of hardware, and no direct comparisons are going to be drawn on any of our graphs. Our goal here is to help you understand where your system is positioned in terms of playing DOOM 3 and to help you choose an upgrade that will make a real difference and offer you the greatest improvement in gameplay for the money spent.