This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
RAM for the RiscPC, while a standard 72pin SIMM, must be bought with a degree of care to avoid potential damage to your machine. EDO RAM, while it will work, is not advised as it may have different power requirements that could be detrimental to your machine.
The RAM required is 70ns (or faster), 72pin, square array (equal number of bits used for row and column addressing), non-parity RAM that supports 'fast page mode' and 'CAS before RAS' refresh. Devices that contain more than 16 memory chips (8 on each side) is not recommended as they may have power requirements above and beyond what the computer can safely supply. Consequently SIMM 'stackers' and 30 to 72pin adaptors are also not advised.
For the more technically inclined out there maximum loads possible are:-
Address 128pF WE 140pF CAS or RAS 59pF Data bus 29pF
Finally, in the maintainers experience anyway, Hitachi parts seem to be fine.