NVIDIA's next-generation graphics card - the GeForce 9800 GTX - is expected in the first quarter of this year and according to Nordic Hardware this card will have a couple of months until competition from ATI will be available.
The dual-GPU R680 will be launched in early 2008 but if the rumour mill is correct we'll have to wait untill summer for the R700 GPU.
The R680 is very much a first test for ATI. The card is kind of a stepping stone on to an even more complex design with the R700. R680 sports two cores on the same PCB. R700 will also have up to two dies per card, but with two cores in each die, bringing the total up to four cores per card. Each die has a single memory interface, which means the cores share the memory buffer.
You might have read elsewhere that the R700 would be delayed until 2009, which was a matter of a gross misinterpretation and simply not true. The slide in question was talking about the gaming platform, Leo, which the R700 will be a apart of, not the GPU itself. R700 is still slated for launch later this year.
The 55nm process used with R700 is a key component for ATI (read AMD). The finer process will (hopefully) lead to higher yields and better margins, meaning bigger income. Something AMD can't get enough of right now. ATI has been fast to adapt new manufacturing processes in the past, but it has probably never been as important as with the R700.