There's a link (although out of stock at the moment =/ ) of a 6800GT which is different from just the 6800 128mb that you were talking about Musjunk, so for $400 you could get the 256mb 6800GT 256-bit which would obviously be way more performance than the 6600GT 128bit 128mb memory card.
Regarding the 6800 128mb 256 bit and the 6600GT 128mb 128bit, you gotta figure (especially if you go through eVGA) that you will be able to overclock it, so really, it seems like the 6600GT even though having half the bit rate, would probably be able to make up for it with faster core/memory speeds and OC'ability although no OC'ability has been guaranteed yet as no one has the AGP 6600GT
especially the doom3 performance is astonishing.
Yes I also read an article where it did quite well, surprisingly well even, the reviewers couldn't believe how far you could up the settings and retain not only good but smooth gameplay.
So in the long run, it would be better to have the 256bit card cause once again if you get eVGA's version you'd get that 6800 up to 450/900 speeds probably as long as you have cool system air, but like I said the 6800 128mb card isn't $400 so if you have $400 to spend then 100% go with eVGA's 6800GT 256mb card