Short addendum: That's what you need, minimum, to have all visual effects. Older video boards (as old as Geforce2 / Geforce4MX) seem to be supported, too, but won't have cube mapped reflections / highlights on top of the normal mapped model as well as no environmental bump mapping effects (which are few, it seems, anyway).
If you want to play DooM3 with surround sound, you really need a > 2GHz class CPU. DooM3 uses an own surround sound algorithm which relies solely on the CPU for mixing.
At least, that's what I'm told. ;) I should get my official copy in about ten days' time, will know more by then.
93,
-Sascha.rb