Looks like ATI has come up with another card:
http://www.techpowerup.com/94814/Rad..._to_RV770.html
This is basically uses a version of the 55nm RV770 core called the RV730CE and uses 128 bit GDDR5.
Perhaps ATI is trying to ditch as many of the RV770 cores it is using ATM and is shifting to only the RV740 and RV790 for its performance cards? Also I could see the production costs for the PCB being less when compared to the HD4830 and perhaps buying GDDR5 for more of its cards means that ATI can pay less for it?