The Inquirer says ATI still hasn't made a final decision of the speed of its Radeon X1800 XT flagship but it will be faster than the anticipated 600MHz for the core and 1400MHz for the memory.
These cards will likely not be available at the (paper) launch date but they should be available two to three weeks after the launch. However, X1800XL cards will be available at the time of launch.
Nvidia release 80 driver should increase the speed for the current G70, Geforce 7800GTX generation and will put some more pressure on ATI. So ATI kind of can clock faster and we already reported that ATI chip can work at even 700+ MHz clock but we don't think that yields will be good at these speeds.