One of the headline features of R600 was its 512-bit wide ring-bus memory controller. This gave unprecedented memory bandwidth and was something ATI was very proud of at the time. However, it was a very silicon-expensive design that didn't use its resources very efficiently. So, for RV770, ATI has gone back to the drawing board and come up with a completely new (though actually fairly conventional) memory controller design.
A central hub allows memory to be accessed from any part of the chip but the actual controllers have been moved to the edge of the chip where they're next to the most memory-bandwidth hungry parts. For low memory usage parts like the CrossfireX interconnect and UVD2 controllers, memory access is handled directly by the hub. By combining this new configuration with the latest GDDR5 memory (at least for the HD 4870) ATI has reduced latency and increased bandwidth to a colossal 192GB/s - nearly twice that of R600.
Another key change in RV770 is the improved AA performance. ATI went back to the drawing board when designing the ROPs and in particular greatly increased dedicated AA hardware. On R600 and RV670, dedicated AA hardware was poor and AA more often than not was performed using the shaders, which had a massive impact on performance.
Energy efficiency has also been improved thanks to clock gating that reduces the clock frequency of the card when not gaming or doing intensive GPGPU work. It would seem these improvements work, as well...
Finally, let's finish our look at the features of RV770 with another one of those lovely overlaid die shots. As with the similiar shot we looked at of GT200, the unmarked section (in this instance coloured in orange) will be a miscellany of other control logic like the thread scheduler and raster setup.