FUD Zilla has some more details about ATI's upcoming RV770 and RV770X2 (R700) GPUs:
The new GPUs are 55nm and they are actually an architecture shrink chip with minor architectural changes. Of course, just as RV670, they do support DirectX 10.1 and at the same time, they will bring some significant performance improvements compared to the previous RV670 generation.
The new RV770 chip will also have great improvements in power consumption. The launch date is late Q2 2008, most likely June, but real volume availability is expected in early Q3 2008, probably July.