I just checked nVidia's website. It says 384MB of memory with a 192-bit interface.
The card has been announced, but it isn't available for sale in the US yet. So, as far as I can find, there is only one way to tell how good it will be:
The 9600GSO is essentially the same card as the 8800GS, nVidia's lowest 8800 series card. So if you look up benchmarks for that, you'll see about how good the GSO is.