According to The Inquirer the ATI R600 GPU will be quite big but it may become the fastest chip to date. According to their sources it may even surpass a clock speed of 1000MHz.
The fastest that Nvidia and ATI can do with 90 nanometres is around 650 to 700MHz and we know that you won't be able to overclock much more with a 90 nanometre chip. This may mean that R600 will be a 65 nanometre component.
The NVIDIA G80 is expected late summer or early autumn, a couple of months before the R600 from ATI. More info over here.