some fresh rumours, Nvidia is working on a new GPU that should counter the HD 6900 series. According to the info, the GTX 580 will show up in December and should be based on the 40nm GF110 GPU, an improved version of the well known and not very succesful GF100 GPU.
The GF110 based GTX 580 should be an improved GF100 GPU with 512 CUDA cores and 128 texture units, which should end up as a decent improvement compared to the GF100. The rumoured card should be equipped with 2GB of GDDR5 memory paired up with a 512-bit memory interface.
The two main questions are the TDP and the actual price of such a card. As you already know, AMD will have its Cayman based HD 6900 cards ready by the end of November, which means that Nvidia will be late and according to what we can guess from these specs, Nvidia's new card will probably end up more expensive. The rumoured TDP places the GTX 580 "close" to GTX 480, and it will need one 6-pin and one 8-pin PCI-Express connector.
The TDP is not a big issue as we already know that Cayman XT will end up with similar "sub-300W" TDP and will need a same 6-pin + 8-pin power connectors. This leaves Nvidia some breathing room, at least if the card ends up to be as good as Nvidia hopes. Of course, this means that AMD will get some competition from the green team and that should hopefully be a good thing for the market as the prices will eventually go down.