RE is massively overhyped, you know
Staying ontopic, I can't remember having any issues whatsoever when I installed and benched a 6600GT (Leadtek) in my SB benchrig (2500K, UD3P, various drivers).
But when I finished benching and just wanted to stress-test the card at stock before selling, I got a screen full of zeros during the driver install. I took as "I've killed it" and since it's no big money, I thrown it away.
Yes, you might as well use a 775 for the 6-series. When I did pretesting on air, going from 4G to 5G SB gave ~1500pts in 01 (mostly in car-tests), ~250pts in 03 (mostly in GT1), ~80pts in 05 (mostly in GT1), ~15pts in 06 (because of CPU test) and no measurable difference in AM3.