When comparing the rev1 to the rev2, the GDDR3 really gives a boost; about 10% to 20% increase. But at some point the GDDR3, when overclocked, stops becoming the bottleneck & the core then becomes the bottleneck.
You all should know that this memory is the first time anyone, outside of a NDA, has seen GDDR3 700Mhz in action. Pretty impressive, though it's slowed down by the 6800Go core. This is what we're probably gonna see on the R520 in May.
What I'm really interested in seeing is the 6600Go in a T&L. If you look at the fillrate of the desktop 6600GT & the desktop 6800 vanilla they are almost the exact same (4000MT/s vs 3900MT/s; it turns into core speed vs quads, really). The 6800 vanilla (12 pipes, 325Mhz core) bests the 6600GT really only when AA & AF are enabled because of it's 256-bit memory interface.
The general consensus is that the 6600GT outdoes the X700, and I imagine a 6600Go with GDDR3 at 1+Ghz would only be slightly slower than the 6800Go rev2, or on par with the 6800Go rev1.6600GT vs 6800 vanilla (12 pipes)