Doom 3 Video Card Help

[mjolnir2k]

I am running a Leadtek GF ti4400 (A250) clocked at 300/600 with an xp2400 (xp 1700 o/c'd) and 512 MB of DDR 3500 ram, 80 GB, and 48X CD with the windows 2000 OS and having major stuttering when trying to run Doom 3.

What is the bottleneck? Video card? I would have thought my trusty GF 4 would be able to handle this...

any thought are appreciated.