So far from what the rumors tell, the R600 could easily be more powerful. It is going to use GDDR4 memory (2.0GHz or higher for sure, they say it can easily reach 2.4GHz and then some), and it's going to be on a 512-bit memory bus. Not to mention the amount of transistors are going to be pretty similar to the 8800GTX.
The R600 is going to dominate the G80 (and even the "G81") in terms of memory, GDDR3 is already at it's limits. The interesting thing is, is that ATI will be using the standard 512/1024 memory sizes because they are going to be using 512bit which is more standard kind of number (not sure if they will go to 1GB on their Radeon series or not), while Nvidia is using their weird memory bus speed on 640MB. The sad part is, is that ignorant buyers are probably going to buy the 8800 just because it will have more memory, and automatically think it's faster. And the whole 7800GTX 512MB phenomenon doesn't help this situation.
The speculation has a firm belief that it will defiantly have 128+ pipes, but the question is if their going to have the unified architecture like the G80's; or if it's going to have physical pipes.
It could easily go either way, that's why I'm waiting this out, and just settle for a 7600GT which has an incredible price/performance ratio.
I'm sure nvidia is probably worried what ATI has under the hood of the R600, so I wouldn't be surprised if their aiming on releasing the "8900" cards probably around Q2 2007; but that is my guess.