From a compute stand point the Turks based Radeon (Aka. Radeon 6570, 6670, 7570, 7670, E6760) would be around 2X as fast as the Hybrid Radeon x1900/2900 found in the xbox and Geforce 7900 based chips found in the PS3.
However, the chips are cheap, low power and are far more efficient than the GPU's found in the Xbox 360 and PS3.
However, expect an increase in performance greater than 2x as the Turks based Radeon has allot more performance saving tricks up it's sleeve than the ancient GPU's found in the Xbox and PS3.
Features such as 3Dc texture compression which allows the GPU to compress Texture Map, Normal Maps, Bump Maps to a ratio of 4:1, which means the chip can get a massive increase in Texture, Bump Map, Normal map quality using the same amount of bandwidth and Ram as the current consoles which typically have to do such tasks on the processor or not at all. (With the exception of Texture compression.)
Texture filtering hardware for floating point textures has been massively improved, essentially Bi-Linear, Tri-Linear, Anisotropic is free.
Tessellation, the Xbox 360 could do it, but at a massive performance cost, it was used sparingly in games and was more or less only used to enhance the geometry in water in some titles like Halo 3, you can really notice the effect in say Viva Pinata too. - The PS3 however lacks this functionality.
More Geometry is never a bad thing. :)
Better hardware scheduler and more/faster/lower latency cache to optimize instruction level parallelism for better performance. I.E. Less shader pipelines are ever idle, another boon over the PS3 is that more pipelines can be focused on a task as the pipelines are all unified.
These are just the performance/efficiency enhancements that are on the "Surface", there has been allot of changes over 7 years in regards to graphics rendering, but I expect several multiples of image quality over the current generation consoles, especially as developers learn all the tricks of the trade with the new hardware.
Also, you can't forget the compute aspects either, CPU not up to the task of doing some Physics calculations? No problem, let the GPU handle it, hence the console would need a less beefier CPU.
Overall, Nintendo also like to build consoles that they can profit from, so going with smaller and more efficient hardware is a wise choice to getting to that goal.
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