This is taken out of the RedBoot code, and is more or less correct:

***The lowest 256MB of address space is configurable based on the value of a configuration register located in the Expansion Bus Controller:

1. When the configuration register is set to logic 1, the Expansion Bus occupies the lowest 256 MB of address space.
2. When the configuration register is set to logic 0, the SDRAM occupies the lowest 256 MB of address.

see page 205 of the Intel developer's manual for the MUCH more detailed version. :)

Note also that default NSLU2 comes with 32MiB even though the range above for memory implies 256MiB space. [http://groups.yahoo.com/group/nslu2-linux/message/1303]