It depends on what Mac mini you have really.
The G4 Mac minis can handle up to 1GB of RAM.
The earlier Intel ones can come preconfigured with up to 2GB of RAM.
Now, seek if you are running Snow Leopard or Snow Leopard Server, you can install up to 8GB of RAM.
If you are running a 2011 Mac mini with Lion or Lion Server, you can install up to 16GB of RAM.