the X850 is not based on the R520 core, it is based the R480 core.
the R480 is almost the same as the R420 (X800) core but designed to get a higher frequency.
there are many arguments for and against the X800's beating 6800's and vice versa, but so far the only thing slightly outstanding is Nvidia's Smartshader 3.0.
and the reason the 6800's perform better in Doom 3 is not because the game was made to take advantage of them. it is written in OpenGL rather than Direct3D, HL2 being written in Direct3D
the Nvidia cards have been designed much better to work with OpenGL than ATI's cards, and they still give comparable performance against ATI's cards in Direct3D
the X850's have an edge over 6800's in Direct3D speed, but still are not so great in OpenGL.
personally I wouldn't give a stuff if any card gets 2-3fps more than another, I like Nvidia's 6800's for being a good reliable card first off, instead of rushing for better ones (with bugs along the way)
Smartshader 3.0 and better OpenGL support are why I like the 6800's
and I am not saying that I dislike ATI either, the X800's are a big improvement over the 9800's, but they could have been designed better. (that is why ATI released the X850)
there's a 6800 Ultra for just over $800 at CPL