This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
For the most part the 8 bit machines were limited to a maximum of around 256k of memory. Although various expansion systems including second processors effectively meant the upper limit was about 1 Meg. (Although I have been told of a second processor with 4 Meg of memory in it. ) For the early 32 bit machines the upper limit currently is 16 Meg of memory. Not all of the range of machines are capable of this however and the list looks something like this:-
A3xx - 8 MB maximum through third party, 1 MB official limit. A4xx - 8 MB maximum through third party, 4 MB official limit. A4xx/I - 8 MB maximum through third party, 4 MB official limit. A3000 - 8 MB maximum through third party, 2 MB official limit. A540/R260/R225 - 16 MB maximum. A5000 - 8 MB maximum through third party, 4 MB official limit. A4 - 4 MB official limit. A30x0 - 4 MB official limit. A4000 - 4 MB official limit. A5000 - 8 MB official limit. (alpha variant of the A5000) A7000 - 128 MB + memory on mother board. (Tops out at 132 MB and requires a 128 MB SIMM to do it.)
Newer RiscPC machines have an official upper limit of 256 Mb, plus 2 MB of VRAM, on all models. However this does require you using 128 MB SIMMS.
If you add the Kinetic StrongARM upgrade (available from Castle Technology) then this upper limit can be expanded further because the Kinetic card contains either 64MB or 128MB SDRam on board. Further information on the Kinetic card is available from Castle Technology (http://www.castle.org.uk).