You'll need (IMHO)...
A good CPU/mobo/ram level (A 2.5 ghz minimum CPU would be my advice, coupled with
at least 512MB of DDR ram)
If you're serious about running the game well, then you need a DX9 compatible video card-Nvidia 6800 GT/Ultra, or Radeon 9800 Pro/x800 would be my advice (I've got an x800 Pro with all 16 pipelines open & Doom 3 flies along quite happily! D )
I also have an Athlon 64 3400+ & a gig of DDR ram!
Good luck.
[Edit]-As mentioned below a beefy powe supply would be essential.
Also-There's no point in upgrading CPU/mobo/ram & then using your old graphics card, as it would just cause you a bottleneck in terms of power (& also vice versa)