It seems that ATI is well aware of the details from NVIDIA's upcomming GT300 series of cards and is prepared to put up a fight.
New information suggests that the RV870 chip will become the Radeon HD 5870 and ATI will be bringing it in a higher clocked dual GPU version as well. ATI will add 50% more shaders to the card going from 800 to 1200, will be increasing the number of texture units up for 48, and doubling the ROps up to 32.
Clock speeds have also been listed too. The Radeon HD 5870 will have a 900MHz core clock and the memory will clock in at 1100MHz. To give the Radeon HD 5870 X2 even more of a boost beyond just dual GPU's, ATI will be bumping both the gpu and memory speeds up and extra 50MHz.
Theoretical performance numbers indicate that the cards will be close to double that of the Radeon HD 4870. ATI is going to need every single bit of that to compete with NVIDIA's GT300.