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.
However, if you want to install RAM yourself, the Mac minis can handle up to 3.16GB of RAM. (This would of course take two 2GB sticks of RAM, with the second one only being partially used.)
The most recent Mac minis (Early 2009) can now handle 8GB of RAM if it is running Snow Leopard.
