ShaderMark 2.0
The same basic trend is visible throughout in ShaderMark 2.0 which scales very very well with GPU core clock (and therefore shader power) increases. 6600GT has around 60% the raw shader performance of 6800 Ultra (in 400MHz mode) and around 75% of the 6800 GT's shader power. All three NV4x-class accelerators utterly outclass the NV35-based 5900 PCX in this full precision DX9.0 PS2.0 shader test.
While not run on the same platform or driver version, you can see an interesting comparison between 6600 GT and plain three quad 6800 here.
