Your Apple Macintosh Performa 640CD DOS Compatible can support up to 52 MB on system board / 32 MB on DOS card of memory. For optimal system performance install the maximum amount of memory in each memory socket, this system comes with standard amount of   4 MB (removable) 4MB non-removable + 4 MB (removable) on DOS card RAM. One or more of the sockets in the system might be already filled with memory. Whenever you upgrade, you can either add memory to one of the open sockets and/or remove memory from a filled socket and replace it with a higher capacity memory module. Select your Memory Upgrade for Apple Macintosh Performa 640CD DOS Compatible.