My god there is a lot of BS in this thread.
Time to make known the facts.
The arcade xboxes were interfaced on a high speed bus from the CPU to a large daughter board. This board has another high speed controller that handles the additional hardware. This board is called the Media Board.
The board contains module connectors for additional ram, connectors for cable looms and additional coinmech logic. The memory types supported by the daughter bord are DIMM, NAND Flash and MASK Flash.
In addition to GDROMs, memory ROMs are also supported.
The picture below shows the connector next to the CPU that supports the high-speed data. This connector was built into the XDK prototype units at the same time the Media Board was being prototyped so this is most likely the way that the media board connected. I cannot confirm this 100%, however the internal support in the kernel for the media board appears to make use of this port.

On another note, ram upgrades on a regular box to 256MB and beyond its fairly easy to impliment. I am already running a 900MHz overclocked xbox and am working on fixing a couple of memory addressing problems. Micron memory is being used as its slightly easier to obtain in the package required. More details to follow in another thread, as its relevance is minimal to this thread.
This hardware has been around for a long time now and illegal versions of the GDROMs have been in circulation for a while. This news is hardly anything new. Some REAL pictures of the Media Board would be interesting, if only to confirm my investigations.
Thanks
Gavroche