Dude don't look too much into BFG's overclock. It is a really weak OC in which you don't even notice any performance gain. It's all a marketing scheme. It's like 20MHz on the core and that's it. That will give you like A point in 3dmark if that.
Yeah all that is ridiculous. SLI would be able to handle it of course, but yeah the 6800Ultra as powerful as it is, can't handle that extreme crap. Like I said Doom3 was made ridiculously intense and full of eye candy. It's not sucking or going slow, it's right on par with what it should be. This is the 2nd time someones posted complaining about it slow, but for god sakes man, take 5 mins to realize what kind of graphics you're trying to pump out of these things.
We're talking the biggest graphical processing leap in years and people aren't satisfied with it......that's just ridiculous.
AGP Aperture is the amount of system RAM that will be used when your Vram is used up. In your cases I doubt it'll make a huge improvement but it might.
If you read up on Doom3 from way back when, they specifically state that Ultra mode was meant for 512mb cards because at the time they were supposed to have been coming out real soon.
With that in mind and using Ultra mode with max res, and full af and aa you can see why it's going 'slow' which is still faster than most peoples here.
Problem solved..theres nothing wrong with the card, it's your expectations thats killing your perception of it.