Your forgetting one very important aspect to this argument. All sources point to the NV40 core as being unable to scale very well. Most reviewers were not able to overclock the card more then 450 MHz, some as little as 425.
You are mixing up the words "scale" and "overclock". As an architecture, the NV GeForce 6 series is designed to "scale" very well, meaning that NV can easily bring out lower priced variants that still have some similar features and properties as the flagship card. ATI's architecture is also very scaleable.
Now, regarding overclocking, way too early to tell. Most of the 6800U cards were A1 samples. Firingsquad had an A2 6800GT, and they were able to overclock their card to 425Mhz core. They were able to overclock the Ultra Extreme card to 475Mhz core.
Also, be careful when talking about the performance of "X800" vs "6800", because there is more than one card that makes up each grouping. Looking at the intial set of reviews, the X800XT is generally slightly faster than the 6800U, and the 6800GT is generally slightly faster than the X800Pro. But then again, it totally depends on the game and the settings, of course.
Here you can see some Shadermark 2.0 tests on the X800XT, 6800U, 6800 GT, and X800Pro:
http://www.firingsquad.com/hardware/geforce_6800_ultra_extreme/page6.asp
