The stock hardware has 32MB of SDRAM.
*WARNING: This WILL DEFINITELY void your NSLU2's warranty!*
It is possible to expand the NSLU2 to at least 64MB by piggybacking another set of SDRAM chips on to the existing ones. All the pins must be wired to the chip below except /CS which needs to be connected to a separate bank enable line from the IXP420.
The bank enable line SDM_CS_N1 is connected to the current bank which leaves SDM_CS_N0 free. SDM_CS_N0 is available from the unconnected pin on PR12 directly below the SDRAM chips.
Both banks must contain identical chips. This means that without removing the existing chips, the maximum amount of memory in total after upgrades is 64MB. If the existing chips were removed however, then a maximum of 256MB is possible with two banks of 128MB. Please see the IXP42x datasheet for more details on allowed configurations.
A new kernel will be necessary to enable usage of the extra memory.
See FatSlug for detailed info.