Till now Doom 3 has the best 3D engine. The animations and textures really put you in the game's environment with the spectacular effects and nightmarish creatures. Truly the Doom 3 offers the spookiest atmosphere ever conceived in a first person shooter.
But let's see the CPU limitation issue with this game. Executing the timedemo demo1 console command I grabbed some fps with both test systems. The first chart shows the game's performance in the following resolutions 800x600, 1024x768, 1280x1024 and 1600x1200 with the high quality settings enabled. The VGA card used was Gainward's Ultra 6800 Ultra.
The Athlon 64 3800+ installed in the Test System 2 really makes a big difference boosting 6800U's performance from 8 to 16 fps. The P4 2.4 CPU installed in Test System 2 held back the VGA card at the 54 fps not letting it perform as good as it could. Now let's if the situation remains the same if the Anisotropic and Anti-Aliasing quality settings are enabled.
Again the Athlon 64 3800+ gave a 5 to 10 fps boost. Here the boost is lower because the Anisotropic and Anti-Aliasing quality settings require more power from VGA card rather than the GPU. But again the 10 fps difference between the 2 systems is worth noting.