The Mac Mini comes with only one memory socket which is populated with 1 256MB 184 Pin PC2700 DIMM from Apple. In order to install more memory, you must remove the factory installed original 256MB..

Standard Memory
256MB (Removable)
Maximum Memory
Memory Expansion (No. of sockets)
1 Expansion Socket
Memory Expansion
1 at a time
Memory Sizes
256MB | 512MB | 1GB
Memory Speed (Type)