Your Apple Power Mac G5 1.8GHz (M9555LL/A) can support up to 4.0 GB of memory. For optimal system performance install the maximum amount of memory in each memory socket, this system comes with standard amount of   256 MB = 128 MB (removable) + 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 Apple Power Mac G5 1.8GHz (M9555LL/A).