Your Compaq Deskpro EN P866 Slim (815e) can support up to 512 MB of memory. For optimal system performance install the maximum amount of memory in each memory socket, this system comes with standard amount of   128 MB (removable) 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 Compaq Deskpro EN P866 Slim (815e).