generally there are people that build stuff, people in the industry and enough information without hard data to make an informed and educated guess. you can to a point say that barcelona will be faster than intels equivelent because the tech details are confirmed. if you read through the anandtech preview on it you can see that almost all the things that were added into conroe to make it a faster and more efficient cpu have been added to the x2's aswell. couple things differently because different architectures take advantage of things slightly differently. but there simply enough info and data to work out it will be faster.
G80 128 shaders, 384mbit bus, 768mb mem, 575/900 clockspeeds.
G90, same but on most likely the 65nm process(G81 same but on 80nm and sounds like its been scraped) and with higher clockspeeds and gddr4 is almost certain.
R600 128 shaders 512mbit bus, 1gig mem, sounds like what, 700-750/1000 clockspeeds(supposed to hit that on 80nm). it doesn't take a genius to make an educated guess that it will be faster. if its 100% certainly on 65nm process maybe clock speeds will be even higher. will that force nvidia to bump their bus up to 512mbit, maybe, will that add to developement time, transistor count and profits, yes. will the G90/81 or ever be hugely faster than either a G80, no, faster but not g70 to g80 type faster. will it be faster than r600, very hard to say, all info points to most likely extremely similar cards. if they perform more than 5-6% differently i'd be shocked.
you could not at all guess if R700 would be faster than a G100 because nothing is known about them, we however know a LOT about the r600.