I understand your frustration, Purge, but I think you have to look at why they are doing this. Doom 3 is made for high-end machines. The technology is the selling point.
Not really frustrated, just annoyed that they didn't show a mid-high-end card; rather than showing $500US+ video cards that are hard to get as of yet.
Not the innovative game play or the fantastic story (try not to laugh, okay), but for the "wow" factor and eye candy. As mentioned elsewhere, Doom 3 is basically a glorified technology demo for the engine itself.
The tech has to be first with iD... they make big bank renting their engine out. Quake3 didn't do so well in the rent dept. There were a few big games, but nothing like Quake2 in terms of adoption.
The hardcore community that cares about these benchmarks are probably all in the 15% you mention..
Bud, I lived in that 15% up until the Xbox was released. Hell, I had the Diamond Monster 3dfx; with Monster II I had the 12MB SLI's. I've dropped big coin on systems in the past; I've grown weary (and wary) of the NEXT BIG GAME (buy buy buy). One game is not enough to justify the cost; otherwise I'd have Steel Battalion right now.
I think your system will do just fine with the game as long as you don't go crazy with the AA/AF.
Being a longtime tech / video game junkie, I know it is. I just wanted to see some numbers.

I am much more interested in HL2 because I think it will be more interesting but I will buy Doom 3, even if it doesn't run as well on my card.
It's all about marketing. Nvidia has a nice little feather in their cap.
Bah. One game using OGL vs. the landslide of DX games. I dunno, it's a small feather as far as I'm concerned. Also, since two of the three console mfgrs are going ATI with their next console, Nvidia is crying in their chocolate milk right now.
I'm with you on the interesting HL2 story; although apparently Doom3 has a story to it and it is told in a much more survival-horror kinda way.
I'm likely going to wait for the Xbox release; I prefer co-op games anyways.