Demo of Doom 3 comes out next month supposely. Here are the requirements for it. Keep in mind that these are probably over exagerrated as is the case has been since quake 1. All of ID Softwares games have ran great on very low end systems.
I fall under the optimal
Minimum:
CPU: 800MHZ ~ 1GHZ
GeForce 3 TI / ATI 9600 Pro
256 MB RAM
Standard:
CPU: 1,7GHZ ~ 2GHZ
GeForce 4 TI 4400/4600 - ATI 9700/Pro
512 MB RAM
Optimal:
CPU: 2,5GHZ ~ 2.8 GHZ
GeForce 5800FX/5900FX - ATI 9800 Pro
786 ~ 1024MB RAM
I fall under the optimal