Various sources have confirmed the existence of a dual G80 card. I wrote about this earlier, here.
The card is real, and a few
people we talked about call it Geforce 8850 GX2. It is of course a dual card with
two G80 chips on the board.
From what we've learned it
should be clocked at higher than 600 MHz core clock, and again an odd memory
number of 896 MB.
One source claims that the
card will have 128 Shaders, but from what we learned before you should expect
96x2 or totally 192 Shader units.
It will certainly be
faster than any overclocked G80 or 8800 GTX today and it should almost coincide with the R600 launch, coming before the end of May.