When you go to a next generation manufacturing process, something that ATI just did with 40nm transition, you have to be aware that you can put a lot more transistors on the same space, compared to an old eg. 55nm process.

RV740 a 40nm chip that will fit the Radeon HD 4770 card and it has 826 million transistors. Radeon HD4870 has 956 million, while Radeon HD 4890 has 959 million transistors and it is quite clear that with 826 million transistors, you have enough space to pack a lot of performance.

The previous generation in the same segment is powered by RV730 XT 55nm chip which has 514 million transistors. If you compare the transistor count of RV740 and RV730XT you can see that the new chip has almost 60 percent more transistors and this resulted in a doubling of the stream processor count from 320 to 640 in RV740 40nm.

ATI pulled a nice trick keeping the 128-bit memory interface, but got twice the bandwidth by using GDDR5 memory.