here's some stuff I found:
What's interesting here is the 9800 model's transistor count, which is nearly identical to the eight-pipe Radeon 9800 XT. But driving 110 million transistors in a laptop chassis, even a big one like the Inspirion XPS, means dialing the engine clock speed down to 350MHz, 22% slower than its predecessor.
X800 XT has 160 million.
so it's obvious the mr9800 isn't using the x800's physical core.
ATI actually leverages X800 technology for MOBILITY RADEON 9800.
This means that the M18 core MOBILITY RADEON 9800 is based around is built on TSMC’s 0.13-micron manufacturing process with low-k dielectric, rather than RADEON 9800’s larger 0.15-micron process. The use of 0.13-micron allows ATI to cram more features into a smaller area and with reduced power requirements than if they would have relied on 0.15-micron, while low-k lets them crank up the clock frequencies without excessive crosstalk, which wastes power and ultimately generates more heat
Six months after ATi launched its Mobility Radeon 9700, the Canadian developers are pulling the next graphics ace from their sleeve - the MR9800. According to ATi, this extremely short development time is the result of bundling its development staff into two development teams, the desktop and the mobile group. While the desktop group focused primarily on the development of the graphics features, the mobile group's assignment was to integrate these features and develop the power management for the individual function blocks.
The Mobility Radeon 9800 will be the Canadians' latest notebook graphics chip controlled by the AGP interface. Its name might lead one to suspect that the graphics core of the MR9800 is a derivative of the Radeon 9800 (R350) or Radeon 9800XT (R360) desktop chip, both of which have eight pixel pipelines and four vertex shaders. However, that is not the case here. The basis for the MR9800 is the graphics core of the X800, i.e. the first graphics chip from ATi with PCI express interface. Why isn't the chip called Mobility Radeon X800, then? Well, ATi decided that only products with the PCI Express Interface may have an "X" in the model name, and the MR9800 is only a graphics processor with an AGP interface.
and compare these design pics