Rys is correct - the jump from R420 to R520 is massive
The entire rendering path is new = physically and conceptually - is brand new
Also, an in depth investigation of the G70/R520 shows that both companies have a very different view of the GPU of tomorow
ATI's focus is on massive maths capability - whereas nVidia seem to believe that texture fetching is everything
Seperating the texture unit from the shader core is a revolutionary concept...
...to then add in separate technology to perform branching in the pixel shader has allowd ATI to offer performance that the top developers like Crytek have labelled "f***ing amazing"
(Rys can furnish you with a precise quote !)
Overall, gamers have never had it so good