AMD acquired ATI back in 2006, but continued to use ATI's brand until the introduction of the Radeon HD 6000 series.
The company realized that the Cypress GPU was incapable of keeping the Stream processors adequately fed. This was inefficient, as all of the cores were powered, but some were idle. The solution was to reduce the number of Stream processors slightly, which resulted in an increase in efficiency and GPU utilization. AMD then added new SIMD engines, though the end result was a 6000-series with fewer resources than its predecessor. This architecture became known as TeraScale 3.
The 6000-series flagship, AMD's Radeon HD 6970, shipped with 1536 Stream processors, 96 TMUs, and 32 ROPs. The 6970's Cypress GPU was manufactured using 40 nm transistors. It also used the same 256-bit bus, but had access to 2 GB of GDDR5. The architectural improvements allowed the 6970 to outperform AMD's previous effort while also being more energy efficient overall.