Doom 3 will be designed to run on systems of a wide variety of specifications... from 600MHz? to 2.53GHz, Doom 3 has to be playable on slower systems otherwise id wont get the desired sales, either that or high end system sales will skyrocket
Depending upon the detail settings and the resolutoin you play the game at, a wide variety of video cards will be supported for the game I am sure. Needing at least a GeForce 3 seems like an absurd requirement for id to make, and 30fps a GeForce 3 will be able to deliver is probably at high resolutions and high detail.
I doubt a R300 or NV30 will be able run Doom3 at 1600 x 1200.
I think the R300 or NV30 will be able to run Doom3 at 1600x1200, at playable framerates as well. Anand tested a pre-production Radeon 9700 with drivers I am sure aren't the best for the board, and UT 2003 was playable (keyword playable) at 1600x1200
with 4x FSAA enabled. Keep in mind that 4x FSAA is a killer on performance as opposed to 2x or none at all. Anand also commented that UT 2003 was
silky smooth at 4X FSAA with the res notched back to 1280x1024...
With that in mind... Doom3 is probably a harder hit on system performance than UT 2003, but UT2003 was playable @ 1600x1200 and 4x FSAA on the 9700. I seriously doubt Doom3 @ 1600x1200 and no FSAA is a harder hit on system performance than that...
The 9700 won't show a huge difference over an 8500 unless you are playing the newest games at the highest resolutions and have FSAA and/or Anisotropic filtering cranked up. A 1600MHz CPU is more than enough especially if you plan on playing that games a the highest resolutions where games hit a bottle-neck at the GPU and not the CPU. Get a faster CPU if you want eleventy-billion fps at 640x480...