I believe most 6800's have 256/256-bit DDR or 128/256-bit GDDR3 memory. Stock 6800 clocks are 350/700.
The 6800GS's have 256/256-bit GDDR3 memory. Stock PCI-e 6800GS clocks are 425/1000, Stock AGP 6800GS clocks are 350/1000.
Both cards have 12 pipelines and the same amount of ROP's and vertex engines I believe.
So with that in mind, you do the math. You obviously can't OC a 6800 by 300-400mhz in memory, especially when the memory is either half of the 6800GS or not GDDR3.