6600 nonGT memory bandwidth=8.8GB/s with 3 vertex pipes and 8 pixel pipes @ 300MHz
9700 Pro memory bandwidth=19.8GB/s with 4 vertex pipes and 8 pixel pipes @ 325MHz
Not too far apart in shader speed. Very far apart in anything which requires lots of memory bandwidth like high-res and FSAA.
Edit: Of course, you can probably overclock the GPU on the 6600 quite a bit but what you really need is more memory bandwidth and you can't overclock your way into near as much of that. A good video board has "balance" between the shader speed and memory bandwidth and an overclocked 6600 has lots of shader speed but is not so great in memory bandwidth.