Looking at Doom3 specs and all that has been said I would recommend the next config recommondations, but still nothing is sure:
Standard stuff: sound, DVD-rom, keayboard, mouse, monitor HDD, etc.
Minimum (only 30 fps, playable but not more than that)
AMD Athlon XP 2000+/Intel P4 2.0 GHz
512MB of DDR-RAM or RD-RAM for P4
ATI Radeon 9600 128 MB or Geforce FX5200 128 MB
Minimum Xtra (a little higher resolution, still low detail and a little higher framarate)
AMD Athlon XP 2800+/Intel P4 2.8 GHz
1024 MB of DDR-RAM/DDR2-RAM
ATI Radeon 9800 128 MB or Geforce FX5700 128 MB
Recommend (fast enough at normal resolutions, pretty good framerates, good to go
)
AMD Athlon 64 2800+ (s754)/3000+ (S939) / Intel Pentium P4 3.0 GHz
1024 MB DDR-RAM/DDR2-RAM
ATI Radeon X800 series 128 MB/Geforce 6600 DDR2 256MB version/6600GT 128 MB (256 MB recommend for better performance)
I used a higher rankl ATI card because of the OpenGL performance difference between GeForce and Radeon.
Indications on Minimum tested on my own PC (specs):
AMD Athlon XP 1800+ oc at almost 2000+ (1633 MHz vs 1667, but faster memorry DDR 286MHz vs DDR 266 MHz)
1024 MB DDRR Ram
ATI Rradeon 9600pro EZ (400 core/400 mem (slower version, between 9600 and 9600pro)) 256 MB.
800x600 low settings in:
Quake4: stable 30 fps
Doom3: stable 40 fps sometimes drops in 30 fps.
Both just playable in single player, but for multiplayer some extra fps is not bad to react in time…