On the initial 8580 models, only 1 MB and 2 MB planar memory modules were available. With the announcement of the 25MHz planars (8580-Axx), 4 MB memory cards became available. However, it has been discovered that they will function on the 20 MHz planars (thanks to Tim Clarke and Joe Kovacs for this discovery).  A summary table of my experiences using various sizes of memory cards in each of the three categories of planars follows:
 

Size of Memory Card	16 MHz	20 MHz	25 MHz
1 MB	OK	OK	NOT OK - 225
2 MB	OK	OK	NOT OK - 225
4 MB	2 MB useable	OK	OK

Note 1: 225 is the POST Error Code resulting from having memory that is too slow.

Note 2: On some 20 MHz planars with 4 MB memory cards on the planar, the IBM diagnostics will occasionally fail, but the memory will still function OK. With the PIICEON 4 MB (70ns.) memory card the IBM diagnostics may also fail. However, both the IBM and PIICEON 4 MB memory cards will function properly and they do pass the Norton Utilities memory diagnostics without any problems. I will be doing more investigation regarding this.