The G80 Ultra is just an overclocked version of the Geforce 8800 GTX. A few of our sources said that Nvidia plans to use faster memory as well, but the chip is the same 90 nanometre marchitecture.
Nvidia plains to launch this new chip in late May and it looks like they will launch it at the end of the month, after they realized how fast the R600XT hardware is.
The good thing is that a lot of birds will talk after Tunisia and Nvidia will have a clear picture of its competition. We highly doubt that R600XT will win every benchmark over Geforce 8800 GTX but it will certainly win some.
We expect that the chip will run around 700 MHz as this is the highest you can overclock the G80 GTX, but you never know.