RV670XT to end up 20+ percent slower
Our sources have confirmed that RV670XT, the card that might be called Radeon HD 3870, should end up more than twenty percent slower than the 8800GT 512MB at its default clock.

ATI's chip simply cannot compete with 112 Shader units at such a high clock but it will sell for some $50 - $60 less to begin with.

RV670PRO will end up even cheaper as it has the hard task of fighting the soon to be launched 8800GT 256MB version.
Fudzilla

RV670PRO to sell for $179
ATI's upcoming RV670PRO will launch with $179 price and as we've said it will fight the soon to be launched Geforce 8800GT 256MB version.

This time around, Nvidia didn't change the clocks of the card, it just decided to put half of the memory and that’s it. ATI on the other hand sells pro as a lower clocked chip that could not hit the super fast 800MHz and makes good use of them.
Fudzilla