There's actually more of a diff. between the 6600 and the 6800 than just a slower chip. The 6800's architecture is vastly more complex than the 6600. Thats why the new Gigabyte SLI card with the dual VPUs used 6600s. The use of dual 6800 processors would have made the card so complex as to be unworkable. IIRC, the 6600 has half the pipes of a 6800. 16 vs 8 in the PCIe version.
The ATI X800 series cards are always going to be a little faster than the 6800's under hard usage. Because of ATI's optimization routines in the X800 series cards, they fudge the Trilinear optimizations for a halfway "Brilinear" mode, and then outright lie about it in their packaging and advertising. Independent testing has already proven it, they just have to admit it. Doesnt necessarily make it BAD, just makes it something different than what they claim. Their dishonesty is a big reason I dont use their stuff. Not that alone, since most businesses fudge words or numbers. But thats a big one.