Just done a comparison on mine and my brothers computers:
Mine:
Athlon XP 2400
Radeon 9000 Pro 64MB
768MB RAM
My Bro's:
Athlon XP 3200
Radeon 9700 Pro 128MB
1GB RAM (Dual Channel)
And it has to be said, forget the extra processing power, this things just eates away at the graphics card (for this comparison anyway). Just like Quake3 and most other FPS's, in Doom3 there are some things you just can't do as good on the slower machine that you can on the faster machine, especially when you are fighting something.
As for how it looks, I suppose it looks alright on the lower detail settings on the slower DX8 cards, however once you see the quality on something faster, you can then easily start to pick apart the lower quality graphics output.
Edit:
As for the Radeon 9000, yeah it is playable at 640x480, but I think that looks a bit poor to be honest, and once it goes to 800x600 it is on the verge of being playable, but just isn't. To be honest, I can't see anyone with a GF4MX really being able to enjoy the game.