Apple Macintosh Performa 6400 Series - Manual (Page 166)

You should have an Apple-certified technician install additional DRAM for
you. However, you may feel comfortable installing DRAM yourself.
m If you want to install additional DRAM yourself, you must purchase the
memory chips, or DIMMs, from an Apple-authorized dealer. Then follow
the instructions in this appendix.
m If you want an Apple-certified technician to install additional DRAM for
you, see the service and support information that came with your computer
for instructions on how to contact an Apple-authorized service provider or
Apple for service.
DRAM configurations
Your computer can use any DRAM configuration with DIMMs of these sizes:
8 MB, 16 MB, 32 MB, or 64 MB. The exact configuration depends on the
density of the DRAM chips that are mounted on the DIMMs. Use only
DIMMs that support a 1K or 2K refresh count. Do not use DIMMs that
support a 4K refresh count.
DRAM DIMM slots (2) for installing
additional DRAM. (You may have an
additional DRAM DIMM installed already.)
Cache slot for installing a High Performance
Module for additional processing power.
166 Appendix B