When I first got Doom 3, my setup was...
Pentium 4, 3.2ghz
1GB of DDR ram,
256MB Radeon 9800 XT graphics card
Sound... Actually, I'm not sure...
Anyways, I played the game at 1024x768 at medium quality with 0 lag, even in more populated or eventful areas.
My current setup-
Pentium 4, 3.2ghz
1GB of DDR ram,
256MB Radeon X850 XT graphics card,
Again with the sound, nothing special
I can now run the game at the highest quality setting with a resolution of 1280x1024. You seriously will want a fast PC for this game, especially so you can experience the amazing full graphics settings. It's just so gorgeous.
As for CJ being able to play it, yeah, probably, just on a bit of a lower setting/resolution, but the point is you'll still get to play it, it's a great game.
Remeeember, always have the lights off, volume up, (EAX is nice), and a 32 inch monitor heightens the effect as well
EDIT: By the way, I actually bought the game

, and the scariest monster imo are imps. Stupid buggers.