RV790 Built on Improved 55 nm Manufacturing Process
It is clear now, that AMD's next performance GPU, the RV790 will be built on the 55 nm manufacturing process. To build on this information, sources tell German website Hardware-Infos that one of the factors that will help the new GPU perform better and run at higher clock-speeds than its predecessor, the RV770 does, is the newer improved 55 nm manufacturing process it will be built on. Going by the foundry-partner codename "55GT", the RV790 may get a little help from its superior silicon fabrication, which invariably makes it more expensive to manufacture.
One of the reasons behind why AMD is starting its 40 nm GPU lineup with a mainstream GPU such as RV740, is that the 40 nm process needs further development by foundry companies. It hasn't developed to the level that safely permits manufacturing high-end GPUs with stellar transistor-counts. For the same reason, NVIDIA's40 nm conquest will be flagged off by the entry-level GT218 GPU. High-density circuits built on the current 40 nm process are known to be very prone to electrical leakage.
RV790 engineering-samples undergoing testing are known to be coming in two flavours according to the source: a base model and an overclocked model. The base model comes with the same exact clock speeds as the RV770XT: 750/900 MHz (core/memory), while the overclocked model is known to come with clock speeds of 850/975 MHz (core/memory). To achieve that 100 MHz increment in core frequency, the voltage is known to be stepped-up from 1.24 to 1.30 V. Products based on this GPU are known to surface by April.
Source: Hardware-Infos