Checking the spreadsheet I've been keeping of my temps, the difference between air cooling with a side fan and stock fan settings (auto fan profile) that I've seen on my XFX 6950 2gb in 3dmark11 are:
Stock clocks, 100% load, extreme setting: 71c GPU, 69c Shaders, room at 26c, fan speed 27%
840/1325, 100% load, extreme setting: 69c GPU, 72c Shaders, room at 25c, fan speed at 32%
With Razor 6970 waterblock, 840/1325, 100% load, extreme setings: 36c GPU, 57c Shaders, room at 24c, fans are PWM controlled to ~20% (approximately 1000rpm, silent on GTX360 radiator)
With Razor 6970 waterblock, 880/1300 (6970 speeds+unlocK), 100% load, extreme settings: 39c GPU, 60c Shaders, 26c room temperature, fans still ~20% on rad
I couldn't stand the 6950 fan when it got over 30% or so since my new case is next to silent (HAF-X). Now with my full custom watercooling setup, I don't have to hear any fans ever. My HDDs are the loudest thing in my computer, but fortunately my OS partition is an SSD, so I don't have to often hear the HDDs wind up.