I've heard that ATI is now already planning the spceification of RV870, the direct successor to the RV770 chip. Current specsifications of RV770 is 480 SPU and 256-bit memory controller (512-bit ring bus). The RV870 chip will include a whooping 1000 SPUs (Stream Processing Units), based on a 45 nm process. The RV870 chip, from what I've seen those guy were discussing, is slated for Q2 2009 release.
While the information out there suggested that RV740 will be released as the mid-range chip. That is in fact not true. Instead, RV730 IS the only mid-range variant of the RV770 chip which will release shortly after the RV770 chip. From what I've heard, the RV740 is in fact a major step to smaller fabrication process, with the whole chip made on the smaller 45 nm process, while the specifications are the same as RV730, so it's good guess that they are pin-compatible to each other in order to save PCB re-designing cost. And the RV740 is scheduled to be released in late 2008 to early 2009.
So, looks like ATI plans to test the potentials of 45 nm and then go full-fledge production on the 45 nm for the graphics chips successing the RV7x0 series.