Just this past Wednesday I switched out the old 6600 GT for a new 6800 GS and the difference was in favor of the GS of course. I could play F.E.A.R. with higher settings and resolution (damn game doesn't have a 1280x1024, can't do 1600x1200 with high-settings).
Z_ONE, that is a good comparison in the cards, as to what they have and what they can push but........
You can see the difference here as well:
LINK.
You can see how they compared to each other and also you can press the " + " in the overclocking part and it'd tell you how much extra juice you'd be getting going from stock clocks to overclock.
I'd take ONE GS over Two GTs..........down the road add another GS for a better gaming experience.......though I'll never go SLI.