Well..it's kinda obvious the 6750 is doing 130MHZ bettter than the 6600, I think that right there is self explanitory!
Your only 170MHZ away from your minumum goal, wich you will need to reach 487FSB, Alot of people have hit 500FSB with your board, so it should be possible.
The higher multiplier of the 6600 would be better if your board was limited to under 450FSB, but that
CPU in general won't clock as high, it's a much older revision of silicone.