Yeah I've heard that too, since its very overclockable.
I have just got the 6800GS today, and I'm impressed with it. its not THE best since I can't run FEAR at full settings really without some jerks, but I know the 7800Gs would cope, even though it has the same number of pipelines.
Its effectiveness works through how many more operations it can carry out per pixel shader, per clock. The 7800GS does 8 operations, while the 6800gs does 4.
Thats where the extra power comes in. Other than that, its pretty much the same.
Oh and of course, you get the transparent antialising possible on the 7 series.