AMD has overhauled their architecture with what I would call evolutionary improvements, it is not a radical redesign. The most notably change is that the shaders are now grouped into two shader partitions of 800 shaders each. On the Radeon HD 5850
two SIMD engines are disabled, which also disables the attached Texture Units, think: "one whole row disabled per shader partition in the diagram".
As before, five Stream Cores per Thread Processor, of which one is a "fat one" and can handle complex shader instructions. They received updates that are required for DX11 and several new instructions that have been identified as "would be nice to have", "would give a nice performance boost". Game developers are usually not concerned with the actual instructions, the game developer writes HLSL shader code which is somewhat similar to C. Then the shader compiler (which is in the ATI driver) turns this high-level code into the instructions that GPU understands.
AMD introduced support for GDDR5 memory with their last generation of GPUs. For you the most important change is that memory clock changes can now happen very quickly, without any flickering. As a result the memory clock can be reduced in idle which shaves a huge wattage off the idle power consumption. Another change is error detection code inside the memory controller which detects errors in the data from memory and automagically corrects it - this will be important for overclocking, read more about it in the Overclocking section of our review.