The minimum GF1 requirement is because the GF1 has a stencil buffer and dot3 blending on hardware. A gf1 will run Doom3 but it will not be nearly fast enough due to GPU, core/ram clocks and less TMU's than later cards.
Entry level (and remember that a game like doom3 is highly dependant on its mappers due to the lighting model) should be round about gf4 if you want anywhere near decent FPS IMO.
You can have 8 opengl lights rendered at once in once pass so long as they do not overlap, 2 overlapping lights alone = 2 passes on a card with one TMU and so on. Then add extra passes for dot3, specular, gloss and luminance map blending.
The Doom3 leak was a memory hog because the model format was ASCII still as it was in the development mode. Expect it to be Binary by release and a lot of textures were still TGA's, expect some what can be jpg's to be jpg's. For people complaining about HDD swapping even with .5 - 1 gig of ram this is why.
Depending on where John "genius" Carmack decides to do his light calcs (CPU or GPU) and how efficent his physics and col det calcs along with all the rest of it (AI, rendering pipelines, raw tris to be rendered blah blah) are you shouldn't need a silly speed CPU. TMU's and raw GFX card b/w are what is really required IMO, but then it is only my opinion and I am often wrong.