The 6850 is about 25-30% faster in DX10 games and 50% faster in DX11 games than a 5770, while using only 10W more at full load.
IIRC the 6850 draws actually around 30W more at full load, not just 10W.
It's idle power consumption is very reasonable.
Idle power draw is about quite the same of the 5770 (around 18W).
IMHO there could also be no need to void the warranty of a HD6850 videocard, fitting an AC S1 Rev. 2: just for example, an MSI Cyclone PE usually tops at about 16/17dB, which you might try to furtherly reduce through a custom fan curve (using Afterburner/Rivatuner).
There's also some HD6870 videocard very quiet, as the MSI R6870 Twin Frozr II OC: it's noticeably more expensive than an oc'ed HD6850 (here about 50-60 euros more), but it gives you much more horsepower than any 6850/5770.