this page is more of a placeholder, to be updated by more knowledgeable people :)
It is more of an analysis of the task than a recipe how to do the job.
I am considering the job (by collecting information and expertise) but have not done it.
Fiddling with the board will void your warranty.
There are basically two ways to add more RAM to the slug.
Solution 1 is based in the fact that the CPU has a separate chip-select (currently unused). It seems safer, because is does not involve removal of the original chips, but is limited to 64MB. The risk is to damage the board using inappropriate tools and/or inadequate expertise.
Solution 2 is based on the fact that there are (currently unused) address lines between the CPU and RAM. It carries more risk (original chips are glued to the board, so physically removing them may damage it; there are two soldering steps). The limit here is 256MB (requires four 64MB chips, from newer PC133? SDRAM add-on modules ??)
Photos of the process are available at http://photos.groups.yahoo.com/group/nslu2-linux/lst?.dir=/Fattened+slug&.src=gr&.order=&.view=t&.done=http%3a//briefcase.yahoo.com/(approve sites)
same as http://ph.groups.yahoo.com/group/nslu2-linux/photos(approve sites)
A good source of pin-compatible chips are older PC133? SDRAM (Synchronous Dynamic RAM) modules, but compatibility must be verified on a case-by-case basis. (FIXME: insert URL's for stock SDRAM chips and replacement ones here).
Both solutions require changes to the bootloader (a separate region in the flash). The APEX bootloader seems to be the (already mature) bootloader of choice.
Changing the bootloader is a non-trivial task. Serial console / JTAG interface are highly recommended, even (most probably) required.